Method of making light redirecting fabric

ABSTRACT

A flexible sheet-form optical system, referenced to as a light redirecting fabric, which has a fabric-like behavior and light redirecting properties. The light redirecting fabric comprises a soft and flexible sheet of optically transmissive material, such as plasticized polyvinyl chloride. A surface of the flexible sheet includes a plurality of parallel slits having spaced-apart walls configured to reflect light by means of a total internal reflection. At least a portion of daylight incident onto the sheet is internally redirected at bend angles greater than the angle of incidence. Disclosed also are a method and apparatus for making the light redirecting fabric. The method includes steps of mechanical slitting of the flexible sheet with a blade, elastic stretch-elongation of the sheet along a direction perpendicular to the slits, and making at least a portion of the sheet elongation irreversible.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 13/855,665, filedon Apr. 2, 2013, now allowed, which claims priority from U.S.provisional application Ser. No. 61/621,332 filed on Apr. 6, 2012,incorporated herein by reference in its entirety, U.S. provisionalapplication Ser. No. 61/691,264 filed on Aug. 21, 2012, incorporatedherein by reference in its entirety, and U.S. provisional applicationSer. No. 61/775,678 filed on Mar. 10, 2013, incorporated herein byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

A portion of the material in this patent document is subject tocopyright protection under the copyright laws of the United States andof other countries. The owner of the copyright rights has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all copyright rights whatsoever. The copyright owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. §1.14.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to improving natural lighting withinbuildings and more particularly to daylight harvesting for buildinginterior illumination. More particularly, this invention relates todaylighting elements of a building such as glazed wall openings,windows, roof windows and skylights, as well as to various devices andglazing structures used for admitting and distributing daylight into theinterior of a building, such as light shelves, light redirecting blindsor louvers, prismatic films and panels, and transparent plates and lightguides employing total internal reflection surfaces.

2. Description of Background Art

Various optical structures for redistributing daylight into buildinginteriors are known. At least some of such prior art devices employplanar transparent plates of glass or plastic materials which includereflective surfaces embedded between the opposing sheet surfaces andconfigured to reflect light by means of a total internal reflection(TIR). The use of TIR structures generally allows for much larger bendangles compared to refractive structures such as prismatic sheets orfilms. Large bend angles are particularly important for redistributingdaylight in the interior of a building so that at least a portion of theincident daylight could be directed towards the upper portions of theinterior, such as the ceiling of a room.

For example, one such light redirecting structure employing internal TIRsurfaces is disclosed in U.S. Pat. No. 737,979 which shows a glass plateincluding a series of slots made in its body. The angle of these slotsis such that daylight coming from any given principal direction fromoutside is reflected from the surface of the slot and is herebyredirected from its original propagation path. Another light redirectingstructure is disclosed in U.S. Pat. No. 6,424,406 which describesoptical diffuser plates made from transparent plastics and employingeither thin strips of another plastic or hollows in the respectiveplates to deflect light.

U.S. Pat. No. 7,416,315 discloses a faceted reflector which includes aplurality of prismatic reflectors embedded in a carrier and reflectinglight by total reflection at a part of the cavity interfaces. In U.S.Pat. No. 6,616,285, total reflection surfaces are formed by merging twooptical bodies each having surface groves which interpenetrate into oneanother when such bodies are placed face-to face. U.S. Pat. No.5,880,886 shows V-section grooves formed in a major face of asubstantially flat and planar optical element. U.S. Pat. No. 4,557,565discloses a planar solid transparent light deflecting panel or plate fortransmitting sunlight into the interior of a building. The panel orplate is formed of a plurality of parallel identically spaced aparttriangular ribs on one face. The ribs have specially selected slopes tototally internally reflect light when such panel or plate is placed overan opening such as window.

On the other hand, various methods of making the light redirecting TIRstructures in such transparent plates have been proposed. For example,U.S. Pat. No. 4,989,952 discloses a method for producing a transparentlight deflecting panel comprising making a series of parallel cuts in asheet of transparent solid material with a laser cutting tool. Suchpanel can be positioned in an opening in the facade of a building todeflect incident daylight towards the ceiling thereby improving thenatural lighting within the building. The transparent sheet is commonlyacrylic and the laser tool is a carbon dioxide (CO₂) laser.

CO₂ laser cutting of rigid PMMA panels has generally demonstrated itsability to form relatively narrow channels suitable for TIR and lightredirection purposes. However, it is fairly difficult to make the widthof the channels less than 150 micrometers (μm) or so, given the finitediameter of the beam of CO₂ lasers, fundamental and practicallimitations of beam-focusing optics and hard-to-control excessivematerial evaporation in the process of ablative material removal. Sincethe minimum attainable width of the laser cut channels relates to theminimum acceptable thickness of the acrylic panel, due to the opticalefficiency and structural considerations, such panels are more practicalin thicknesses of 6 mm or more. However, a 6-mm acrylic panel can befairly heavy for the typical sizes of wall windows which can translateinto substantial material costs and difficulties in fixing the panel ina suspended position near the window.

U.S. Pat. No. 6,580,559 describes a method of forming internal TIRstructures in transparent panels made from glass-like thermoplasticmaterial such as PMMA by inducing parallel crazes in the thermoplasticmaterial. An organic solvent is applied on the panel surface while atensile stress is applied to a panel which results in generation ofwedge-shaped deformations (crazes) which propagate within the material.However, such method of forming internal TIR structures offers littlecontrol over the spacing, depth and extent of the crazes, as well as cansubstantially compromise the structural integrity or rigidity of thepanel.

Mechanical surface slitting of a sheet-form material using razor-sharpblades can be one way of making TIR channels or facets within the bulkmaterial of an optically clear sheet. However, slitting of hard andbrittle optical plastics such as polycarbonate, polystyrene, rigid PVCor polycarbonate with a blade is generally unfeasible considering thelack of sufficient plasticity of such materials and is usually ruled outfor daylighting devices employing TIR surfaces. The blade penetrationinto such materials induces material chipping and/or smearing whichmakes it nearly impossible to form a smooth and narrow cut suitable forTIR functionality even assuming the ideal sharpness of the blade.Additionally, the blade thickness brings its own limitations on theminimum width of the cut in a rigid material.

The use of rigid transparent plates or panels as a substrate in whichTIR surfaces can be formed is also deficient in that such devices areinherently inflexible which makes them difficult or impractical to beutilized in lightweight and compact daylighting systems. It is alsodifficult or impractical to incorporate rigid sheets or plates inretractable window coverings or use them for lamination onto a windowpane or other substrates used in glazing.

Many daylighting systems would benefit from employing a sheet-form lightredirecting TIR structure that can be sufficiently thin and flexible,resembling the behavior of a fabric, and that can be bent to tight radiior laminated onto a surface. Furthermore, many daylighting applicationswould also benefit from an efficient and low-cost method of making suchflexible TIR sheet-form structures.

Accordingly, prior efforts have failed to provide a practical andcost-effective solution for admitting daylight into building interiorsand providing uniform and efficient natural illumination using arelatively thin sheet material. These needs and others are met withinthe present invention, which provides an improved sheet-form structurefor illuminating building interiors with sunlight and also provides amethod of making the same. The improved sheet-form structure employsinternal TIR surfaces to efficiently redirect light and is also thin,flexible and has a fabric-like behavior, all of which finds utility invarious daylighting devices and systems.

BRIEF SUMMARY OF THE INVENTION

The present invention solves a number of daylight harvesting anddistribution problems within a thin and flexible sheet-form opticalsystem which has a fabric-like behavior. Apparatus and method aredescribed for directing and distributing daylight within buildinginterior using an optically clear flexible sheet material in which lightredirecting functionality is provided by an array of deep and narrowslits formed in a surface of the material. Daylight passes through thesheet-form material configured with internal TIR surfaces and isredirected into building interior at high deflection angles with respectto the incident direction.

A light redirecting fabric including a soft and flexible sheet ofoptically clear material is described. The flexible sheet comprises aparallel array of deep and narrow slits formed in a broad-area surfaceof the sheet material. Each slit comprises two parallel or near-parallelwalls permanently separated from each other by a layer of air. At leastone of the walls of the slit is configured to reflect light by means oftotal internal reflection (TIR) in specular or near-specular regime. Thesurface of the wall may have a surface roughness which does not exceed apredetermined value.

A method of making the light redirecting fabric and several embodimentsof an apparatus for implementing such method are also described. Themethod includes a step of mechanical slitting a broad-area surface ofthe flexible sheet using a sharp blade or razor. The method furtherincludes a step of stretching the flexible sheet along a direction whichis perpendicular to the longitudinal axis of the slits. Such stretchingresults in the elongation of sheet material in the areas of the slits.The method of making the light redirecting fabric further includes astep of irreversible separation of the opposing walls of the slits.

The invention is amenable to being embodied in a number of ways,including but not limited to the following descriptions.

At least one embodiment of the invention is configured as a lightredirecting fabric for illuminating a building interior with sunlight,comprising: (1) a soft and flexible sheet of optically transmissivematerial and (2) at least a first plurality of parallel slits formed ina surface of said sheet. Each of the slits includes a first wall and anopposing second wall extending generally parallel to the first wall. Thefirst wall of each slit is spaced apart from the respective second wallby a predetermined distance and at least one of the first and secondwalls is configured for reflecting light by means of a total internalreflection. The device operates in response to daylight received on asurface of the sheet being angularly redirected by the plurality ofslits and distributed from the opposing surface of the sheet into thebuilding interior. In at least one implementation, the light redirectingfabric is configured to intercept at least a portion of the incidentbeam and redirect such portion at a bend angle which is about two timesof the incidence angle. In at least one implementation, the bend anglesmay exceed 90° for at least some incidence angles onto the surface ofthe sheet. In at least one implementation, the sheet of lightredirecting fabric is selected so that a parallel beam of light passingthrough the sheet can be split into two or more beams propagatinggenerally into opposing directions from a normal to a sheet surface. Indifferent implementations, either broad-area surface of the sheet may beconfigured for light input while the opposing surface may be configuredfor light output.

In at least one implementation, the soft and flexible sheet of opticallytransmissive material is made from a plasticized polyvinyl chloride(PVC). Such material is preferably optically clear or translucent andhas a sheet thickness between 0.5 mm to 3.2 mm. In at least oneimplementation, the sheet of the light redirecting fabric has agenerally square or rectangular shape defined by a first broad-areasurface and an opposing broad-area surface. In at least oneimplementation, the flexible sheet is configured for a generallyunimpeded light passage along at least a direction which isperpendicular or near-perpendicular to the surface of the sheet. In atleast one implementation, the flexible sheet is configured for asee-through appearance. In at least one implementation, the flexiblesheet is configured for a see-through appearance and a privacy function.

Each slit formed in the flexible sheet is preferably deep and narrowand, according to an aspect, forms a high aspect ratio void in a surfaceof the sheet. In at least one implementation, each slit 6 extendsthrough more half of the thickness of the sheet. In at least oneimplementation, each slit does not reach at the opposing surface of thesheet and extends into the sheet material to a depth which is 50% to 95%of the thickness of the sheet.

In at least one implementation, the first and second walls of the slitsare separated from each other by a layer of ambient air. According to anaspect, each of the slit walls defines a TIR interface between thematerial of the sheet and the ambient air. According to another aspectthe plurality of the slits may be viewed as a plurality of thin,two-sided TIR reflectors formed between opposing surfaces of the lightredirecting fabric. In at least one implementation, at least one of thefirst and second walls of each slit is configured to reflect light by atotal internal reflection in a specular or near-specular regime.

In at least one implementation, the opposing walls of each slit aregenerally planar and extending generally parallel to each other and alsoperpendicular to a surface of the flexible sheet within a predeterminedangular accuracy. In various implementations, the parallelism of theopposing walls of each slit should be within a few degrees, moreparticularly, within about six degrees, within two degrees and withinone degree. In at least one implementation, each slit extends into thematerial of the flexible sheet generally perpendicular to the surface ofthe sheet. In at least one implementation, at least some slits extendinto the material of the flexible sheet at a constant angle with respectto a surface normal of the sheet. In at least one implementation, atleast some slits extend into the material of the flexible sheet at avariable angle with respect to a surface normal of the sheet. In atleast one implementation, the distance between the opposing walls of theslits are preferably within a 5 μm to 100 μm range, and more preferably,within a 10 μm to 50 μm range.

In at least one implementation, the plurality of the slits is configuredto provide a controlled spread of the parallel beam incident onto thelight input surface of the flexible sheet.

In at least one implementation, each of the opposing broad-area surfacesof the flexible sheet is generally smooth. In at least oneimplementation, at least one of the opposing broad-area surfacesincludes a plurality of surface relief features. In at least oneimplementation, such surface relief features are configured fordiffusing light passing through the respective surface. In at least oneimplementation, the surface relief features are selected from the groupof elements consisting of prism arrays, arrays of prisms, prismaticgrooves, lens arrays, engineered surfaces, and surface relief typescommonly referred to as “frosted-glass”, “prismatic”, “sanded”,“pebble”, “ice”, “matte”, “microprism”, and “microlens”. In at least oneimplementation, at least one of the surfaces of the flexible sheetincludes an image print or pattern. Such print or pattern may have lightdiffusing and/or decorative functions.

In at least one implementation, at least a portion of the surface of thewalls of the slits has a non-negligible surface roughness. According toan aspect, the surface roughness may be represented by a plurality ofsurface relief features formed in the respective wall of the slit.According to an aspect, the surface roughness may be characterized by aroot-mean-square (RMS) roughness parameter R_(q). In variousimplementations of light redirecting fabric, the RMS roughness parameterR_(q). is selected to be within a predefined range. In oneimplementation, it may be preferred that the RMS surface roughness R_(q)of at least a substantial portion of the surface of the slits is in therange between 0.01 μm (10 nm) and 0.1 μm (100 nm). In an alternativeimplementation, it is preferred that R_(q) is generally less than 0.05μm and even more preferred that R_(q) is less than 0.03 μm. According toan aspect, it may be preferred that the sampling lengths for measuringsuch R_(q) parameter of the surface of slits are within the range of 20μm to 100 μm. According to another aspect, it may be preferred that thesurface profile roughness Redoes not exceed the prescribed values alonga line which is either perpendicular to the longitudinal axis of therespective slit or is disposed at a relatively low angle with respect tosuch axis.

In at least one implementation, each of the slits is formed mechanicallyusing a sharp blade or razor. In one implementation, the radius of thecurvature of the tip of such blade or razor in a transversalcross-section should preferably be on a sub-micron scale. In oneimplementation, it may be further preferred that such radius ofcurvature is less than 50 nanometers.

In at least one implementation, each of the slits formed in the flexiblesheet extends substantially through the entire width of the sheetmaterial. In at least one implementation, the length of each of theslits formed in the flexible sheet is less than the width of the sheetmaterial. In at least one implementation, the plurality of slits isarranged in staggered rows and columns where each row is shiftedrelatively to the adjacent rows. In each row, the adjacent slits areseparated by a spacing area of the uncut bulk sheet material.

In at least one implementation, the soft and flexible sheet furthercomprises a second plurality of parallel slits formed in a surface ofthe sheet and extending generally perpendicular to the first pluralityof parallel slits. In alternative implementations, the second pluralityof parallel slits can be formed in either the same surface as said firstplurality of parallel slits or in the opposing surface. According to anaspect, the two pluralities of slits crossed at the right angle to eachother forms a plurality of light-channeling cells. In at least oneimplementation, each of the light-channeling cells is configured toredirect and redistribute at least some off-axis rays entering onto asurface of the sheet.

The light redirecting fabric may be operated in a number of ways. In atleast one implementation, a sheet of the light redirecting fabric issuspended within or in proximity of an opening in a building façade. Indifferent implementations, such sheet may be positioned in a horizontalorientation, in a vertical orientation or at an angle with respect tothe horizontal plane. In at least one implementation, a sheet of thelight redirecting fabric is laminated onto a surface of a window pane.In at least one implementation, a sheet of the light redirecting fabricis positioned in an opening of a roof window or a skylight. In at leastone implementation, a sheet of the light redirecting fabric isconfigured to redirect daylight onto a ceiling of a room. In at leastone implementation, a sheet of the light redirecting fabric is stretchedalong a direction which is perpendicular to the longitudinal axis of thelinear slits.

In at least one implementation, the surface of the sheet in which theplurality of slits is formed is facing the building interior. In atleast one implementation, such surface is facing the source of daylight.In at least one implementation, the flexible sheet is positioned withina building interior above the eye height of the building occupants.

In at least one implementation, the flexible sheet includes one or moreadditional layers. Such layers may include light diffusing features,light filtering features, protective layers, or color filtering or tintfeatures.

In at least one implementation, the sheet is adapted for being retainedin either a planar configuration or in bent and/or rolledconfigurations. In at least one implementation, the sheet is adapted forbeing retained in a stretched state.

At least one embodiment of the invention is configured as a sheet-formlight-redirecting structure employing a plurality of internal opticalinterfaces within the structure. Each of the optical interfaces isconfigured to reflect light by means of TIR and is defined by a surfacewhich includes surface irregularities and can be characterized by an RMSsurface profile roughness parameter. The RMS surface profile roughnessparameter should preferably be measured along a reference line which isparallel or near-parallel to the plane of reflection. The roughnessparameter of at least a substantial part of the surface area should begenerally between 10 nanometers and 100 nanometers at a sampling lengthbetween 20 and 100 micrometers. In at least one implementation, each ofthe surfaces is generally planar and extends generally perpendicular tothe prevailing plane of the sheet-form structure. In at least oneimplementation, each of the surfaces is generally planar and is disposedat an angle other than normal to the prevailing plane of the sheet-formstructure. In at least one implementation, at least some of the surfaceshave a curvilinear shape. According to an aspect, each of the TIRsurfaces within the structure may be configured to reflect light in aspecular or near specular while having a non-negligible surfaceroughness. According to another aspect, the light redirecting structuremay be configured to redirect light by means of TIR while providinglight diffusing function.

At least one embodiment of the invention is configured as a method forredirecting light comprising: (a) propagating the light in an opticallytransmissive material; (b) reflecting light by a total internalreflection at a plurality of optically-irregular surfaces distributedwithin the material; and (c) extracting the reflected light from a lightoutput surface of the material. In at least one implementation, an RMSsurface profile roughness parameter characterizing the relief of theoptically-irregular surfaces is generally between 10 nanometers and 100nanometers at a sampling length between 20 and 100 micrometers. In atleast one implementation, the light output surface includes a pluralityof surface relief features.

At least one embodiment of the invention is configured as a method formaking a light-redirecting fabric from a flexible sheet of opticallyclear material comprising: (a) a step of forming a parallel array ofslits in a surface of the flexible sheet by means of at least one bladeor razor; (b) a step of elastic elongation of the sheet along adirection perpendicular to the slits; and (c) a step of making at leasta portion of the elongation irreversible. In at least oneimplementation, the step of making at least a portion of the elongationirreversible includes elongation of the sheet in a plastic deformationmode. In at least one implementation of the method, the step of makingat least a portion of the elongation irreversible includes annealing ofthe sheet at an elevated temperature. In at least one implementation,the elevated temperatures may be in the range between 60° C. and 150° C.In at least one implementation of the method, the blade or razor is of arotary type. In at least one implementation, the method for making thelight-redirecting fabric further includes a step of heating at least asurface portion of the sheet to a predetermined temperature. In at leastone implementation, the relative irreversible elongation of the sheetdoes not exceed 10% of the original sheet length. In at least oneimplementation, the relative irreversible elongation of the sheet doesnot exceed 5% of the original sheet length. In at least oneimplementation, the elongation of the material in the areas of the slitsduring the step of making at least a portion of the elongationirreversible exceeds 100%.

At least one embodiment of the invention is configured as an apparatusfor making a light-redirecting fabric from a flexible sheet of opticallyclear material comprising: (a) means for stretching the flexible sheetalong a first direction and (b) means for mechanical slitting of asurface of the sheet along a second direction. In at least oneimplementation, the apparatus for making the light-redirecting fabricfurther includes a heat source configured for heating at least a portionof the sheet to a predetermined temperature. In at least oneimplementation, the means for mechanical slitting includes at least oneslitting blade or razor. According to an aspect, each of the slittingblades or razors is configured to form slits which surface has aroughness below a predetermined threshold value. In at least oneimplementation, the means for mechanical slitting includes at least onerotary slitting blade. In at least one implementation, the rotary bladehas the thickness of 0.3 mm or less. In at least one implementation, themeans for mechanical slitting includes a plurality of blades arranged inone or more packs. The blades may be arranged within each pack at aconstant spacing distance which is a predetermined factor of the nominalslit spacing.

In at least one implementation, the slitting blade or the pack ofslitting blades is provided with a linear motion system which prevailingmotion axis is positioned along the intended slitting direction. In atleast one implementation, each of the blades or razors is positionedperpendicular to the surface of the sheet. In at least oneimplementation, one or more slitting blade or razor is positioned at anangle with respect to a normal to the surface of the sheet. In at leastone implementation, the apparatus for making a light-redirecting fabricis provided with means for varying the tilt of the blade with respect tothe surface of the sheet.

In at least one implementation, the means for stretching the flexiblesheet include at least one roller or roll. In at least oneimplementation, the means for stretching the flexible sheet include atleast two rollers disposed parallel to each other. In oneimplementation, the two rollers are made movable with respect to eachother. In at least one implementation, at least one of the rollers isprovided with a drive configured for rotating the roller in predefinedangular increments. In an alternative implementation, the means forstretching the flexible sheet include one or more clamping bars.

In at least one implementation of the apparatus for making alight-redirecting fabric, the first and second directions are generallyparallel to each other. In at least one implementation of the apparatusfor making a light-redirecting fabric, the first and second directionsare generally perpendicular to each other.

The present invention provides a number of beneficial elements which canbe implemented either separately or in any desired combination withoutdeparting from the present teachings.

An element of the invention is an apparatus and method of collectinglight over a given area and traveling in a first direction, andredirecting at least a portion of such light into a second directionwhich can have any desired angular relationship with the firstdirection.

Another element of the invention is the use of a soft and flexible sheetof an optically transmissive material which can also be transparent ortranslucent.

Another element of the invention is the use of plasticized polyvinylchloride (PVC) material in a sheet-form which can have a fabric-likebehavior.

Another element of the invention is the use of a thin and flexible sheetwhich has a light redirecting function and which can be positionedwithin or in a vicinity of an opening in a building façade.

Another element of the invention is the use of a thin and flexible sheetwhich has a light redirecting function and which can be positionedwithin or at the exit aperture of a daylighting device or structure.

Another element of the invention is the inclusion of distributeddeflecting means within the interior of the soft and flexible sheet.

Another element of the invention is the use of reflective meansoperating within the sheet and configured to reflect light by means of atotal internal reflection.

Another element of the invention is the use of a parallel array of slitsformed in a surface of the sheet to make the reflective means.

Another element of the invention is the use of slits which opposingwalls are spaced apart from each other and separated by a layer ofambient air.

Another element of the invention is the use of a linear array of slitswhich span the surface of the sheet, or a portion thereof.

Another element of the invention is the use of total internal reflectionsurfaces which are irregular by optical standards.

Another element of the invention is the inclusion of surface relieffeatures into at least one surface of the sheet to extract and/ordiffuse the redirected light.

Another element of the invention is an apparatus for making a lightredirecting system from a sheet of soft and flexible material.

Another element of the invention is the use of a blade or razor to formlight reflecting surfaces in a sheet of optically transmissive materialby means of creating linear slits.

Another element of the invention is the use of a blade or razor with ahigh edge quality to minimize the surface roughness of the slits.

Another element of the invention is the use of a blade or razor having acutting edge with a tip radius (in a cross-section) on a submicronscale.

Another element of the invention is the use of a mechanical stretchingof the sheet along a direction perpendicular to the slits to separatethe opposing walls of the slits from each other.

Another element of the invention is the use of heat to aid theseparation of the opposing walls of the slits.

A still further element of the invention is the use of slits is asurface of the sheet to form deep and narrow voids in the material ofthe sheet.

Further elements of the invention will be brought out in the followingportions of the specification, wherein the detailed description is forthe purpose of fully disclosing preferred embodiments of the inventionwithout placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a schematic perspective view of a rectangular sheet of a lightredirecting fabric, according to at least one embodiment of the presentinvention.

FIG. 2 is a schematic perspective view of a portion of a lightredirecting fabric, showing a plurality of slits formed in a surface ofa flexible sheet material, according to at least one embodiment of thepresent invention.

FIG. 3 is a schematic top view of a flexible sheet having a plurality ofslits extending parallel to a reference line, according to at least oneembodiment of the present invention.

FIG. 4 is a schematic view showing the operation of a light redirectingsheet of a light redirecting fabric disposed in a vertical orientationin an opening of a building facade, according to at least one embodimentof the present invention.

FIG. 5 is a schematic view showing the operation of a light redirectingsheet of a light redirecting fabric disposed an angle to a verticaldirection in a vicinity of a roof window or skylight of a building,according to at least one embodiment of the present invention.

FIG. 6 is a schematic cross section view and raytracing of a lightredirecting fabric portion, according to at least one embodiment of thepresent invention.

FIG. 7 is a schematic view showing raytracing diagrams and illustratingvarious regimes of light reflection from a surface.

FIG. 8 is a schematic view and raytracing of a portion of a lightredirecting fabric, showing light rays reflected from a wall of a slitin various reflection regimes, according to at least one embodiment ofthe present invention.

FIG. 9 is a graph showing calculated exemplary dependencies of specularreflectivity from a root-mean-square roughness of a surface.

FIG. 10 is a schematic perspective view of a surface portionillustrating different amplitude of surface roughness in orthogonalplanes and also showing a ray reflection in a specular regime.

FIG. 11 is a schematic view illustrating a method of making a lightredirecting fabric, showing steps of slitting a sheet of flexiblematerial using a blade or razor and slit walls separation, according toat least one embodiment of the present invention.

FIG. 12 is a schematic view showing further details of individual stepsof a method of making a light redirecting fabric, according to at leastone embodiment of the present invention.

FIG. 13 is a schematic graph illustrating a stress-strain curve of aplastic material.

FIG. 14 is a schematic perspective view of an exemplary apparatus formaking a light redirecting fabric, showing various steps of processing aflexible sheet material, according to at least one embodiment of thepresent invention.

FIG. 15 is an exemplary measured surface roughness profile of a slitformed in a flexible PVC sheet.

FIG. 16 is a schematic perspective view illustrating a variation of anapparatus and method of making a light redirecting fabric, showing acontinuous slitting of a flexible sheet, according to at least oneembodiment of the present invention.

FIG. 17 is a schematic view illustrating an apparatus and a method formaking a light redirecting fabric using a plurality of blades, accordingto at least one embodiment of the present invention.

FIG. 18 is a schematic view illustrating different steps of a method ofmaking a portion of a light redirecting fabric using a plurality ofspaced-apart blades, according to at least one embodiment of the presentinvention.

FIG. 19 is a schematic cross section view and raytracing of a portion ofa light redirecting fabric, showing a plurality of parallel slits formedin a light input surface of an optically clear sheet material, accordingto at least one embodiment of the present invention.

FIG. 20 is a schematic cross section view and raytracing of a portion ofa light redirecting fabric, showing a textured light output surface,according to at least one embodiment of the present invention.

FIG. 21 is a schematic view and detailed raytracing of a portion of alight redirecting fabric, showing a textured light output surface of anoptically transmissive body and further showing slits formed in asurface of the sheet and having surface relief features, according to atleast one embodiment of the present invention.

FIG. 22 is a schematic cross section view of a light redirecting fabricportion comprising additional layers of optically transmissivematerials, according to at least one embodiment of the presentinvention.

FIG. 23 is a schematic view of a portion of a sheet of light redirectingfabric, showing a plurality of slits formed at a constant angle withrespect to a perpendicular to the sheet surface, according to at leastone embodiment of the present invention.

FIG. 24 is a schematic view of a portion of a sheet of a lightredirecting fabric, showing a plurality of slits formed at a variableangle with respect to a perpendicular to the sheet surface, according toat least one embodiment of the present invention.

FIG. 25 is a schematic view illustrating a step of a method of makinglight redirecting fabric, showing a slitting blade disposed in an offsetposition with respect to a roller, according to at least one embodimentof the present invention.

FIG. 26 is a schematic top view of a rectangular sheet of a lightredirecting fabric, showing a staggered arrangement of slits in asurface, according to at least one embodiment of the present invention.

FIG. 27 is a schematic top view of a rectangular sheet of a lightredirecting fabric including a plurality of slits arranged in twoperpendicular arrays, according to at least one embodiment of thepresent invention.

FIG. 28 is a schematic view and raytracing of a portion of a lightredirecting fabric including a light-channeling cell, showing variousray paths through such cell, according to at least one embodiment of thepresent invention.

FIG. 29 is a schematic perspective view and raytracing of a portion of alight redirecting fabric, showing an exemplary path of a ray reflectingfrom multiple walls of a light-channeling cell, according to at leastone embodiment of the present invention.

FIG. 30 is a schematic view and raytracing diagram illustrating theoperation of a conventional diffuser in a skylight.

FIG. 31 is a schematic view and raytracing diagram showing a lightredirecting fabric incorporated into a skylight, according to at leastone embodiment of the present invention.

FIG. 32 is a schematic perspective view of a window, showing lightredirecting fabric laminated onto a window pane, according to at leastone embodiment of the present invention.

FIG. 33 is a schematic cross section view of a portion of a lightredirecting fabric laminated onto a window pane, according to at leastone embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring more specifically to the drawings, for illustrative purposesthe present invention is embodied in the apparatus and method generallyshown in the preceding figures. It will be appreciated that theapparatus and method may vary as to configuration and as to details ofthe parts without departing from the basic concepts as disclosed herein.Furthermore, elements represented in one embodiment as taught herein areapplicable without limitation to other embodiments taught herein, and incombination with those embodiments and what is known in the art.

The present invention particularly seeks to provide illuminationcomponents capable of receiving daylight entering building interiorsthrough various openings, such as wall windows, roof windows, doors andskylights and redistributing such daylight for improved daylightingefficiency, more uniform spatial distribution and reduced glare.Daylight is generally referred to both the direct and indirect sunlightstriking the respective openings in buildings during the daytime. Thedirect sunlight represents a quasi-parallel beam from the sun and theindirect sunlight represents the diffuse solar radiation scattered outof the direct beam by the sky and various outdoor objects. While theoperation of the following embodiments is primarily described by exampleof the direct sunlight, it should be understood that this invention mayalso be applied for admitting and redistributing the diffuse componentof sunlight within a building interior.

The following embodiments of the present invention are generallydirected to a flexible sheet-form optical article or system which may beconfigurable for light redirecting operation in response to the incidentdaylight and further configurable for a fabric-like behavior in responseto its handling. Therefore, such optical article or system ishereinafter referred to as a “light redirecting fabric”. It is noted,however, that the term “fabric” should be understood broadly, withoutparticular regard to specific types of fabric produced by weaving orknitting textile fibers, as it mostly reflects the relatively soft andflexible feel of such sheet-form optical article and not its texture orcomposition. Accordingly, the term “fabric” should not be construed aslimiting this invention in any way. As it will be understood from thefollowing description, the term “fabric” may be particularly relevant inthe context of the present invention considering that the disclosedoptical articles and systems may be formed from various unwoven soft andflexible materials, such as, for example, sheets of flexible vinyl, thatare commonly referred to as “fabric” in the art.

FIG. 1 illustrates a first embodiment 2 of a light redirecting fabric.In this embodiment, light redirecting fabric 2 is formed by a relativelythin, soft and flexible sheet 4 of optically transmissive material. Thematerial should preferably be clear or translucent so that sheet 4 cantransmit light incident onto its surface without substantial losses.Sheet 4 has a generally rectangular shape defined by a first broad-areasurface 10 and an opposing broad-area surface 12. The material of sheet4 is exemplified by optically transmissive, plasticized polyvinylchloride (PVC). The plasticized polyvinyl chloride is commonly referredto as soft vinyl, flexible vinyl, soft PVC, flexible PVC, plasticizedPVC, and PVC-P. Such terms are often used interchangeably in the artwith respect to plasticized polyvinyl chloride so that they may also beused interchangeably in the following description in reference to thismaterial.

In contrast to the rigid (unplasticized) PVC which is a semi-crystallinepolymer with a relatively high glass transition temperature and tensilemodulus, soft PVC contains considerable amounts of plasticizing entitieswhich lower its tensile modulus and also lower the glass transitiontemperature of the material to below room temperature which makes softPVC to feel slightly rubbery.

There are currently hundreds of known PVC plasticizers and at leastseveral dozens of suppliers. Some of the common chemical families usedin plasticizing PVC include dialkyl orthophthalates, alkyl benzylphthalates, dialkyl tere-phthalates, epoxides, aliphatic carboxylicdiesters, polyester-type polymerics, phosphate esters, trimellitateesters, benzoate and dibenzoate esters, alkyl sulphonic esters of phenoland cresol, and miscellaneous-type plasticizers.

Other entities commonly added to PVC may also include stabilizers,pigments, fillers, lubricants, fire retardants, UV-stabilizers,antistatic additives, and the like. While pigments are often used invarious PVC-based products, a pigment-free, clear formulation of PVC-Pis preferred for sheet 4.

Clear and flexible PVC-P materials which may be suitable for sheet 4 arecommonly available in the form of large-area sheets or rolls. Suitablematerial thicknesses may include the range from a fraction of amillimeter to several millimeters.

By way of example and not limitation, sheet 4 may be made from one ofthe several types of commercially available clear vinyl sheeting usedfor window glazing, tent windows, awnings, boat windows, totallytransparent or slightly tinted curtains and the like. Since theplasticized vinyl available in thin flexible sheets possessesrubber-like flexibility, can be sewn and resembles fabric or cloth inits behavior, it is also often referred to as vinyl fabric. Sheets madefrom clear and flexible vinyl fabric and processed according to theteachings of at least some embodiments of the present invention may thusalso exhibit the fabric-like feel and behavior. Accordingly, in thecontext of the present invention, the term “fabric” is particularlydirected to include a flexible sheetform of soft and tough material,such as plasticized PVC sheeting, and more particularly include softvinyl sheet material in the thickness range from 0.5 mm to 3.2 mm.

For instance, sheet 4 can be cut to size from a 30-, 40-, or 60-gage(0.8 mm, 1 mm and 1.5 mm, respectively) clear vinyl fabric commerciallyavailable by the yard or by the roll from online retailers such asMyTarp.com, onlinefabricstore.net, or fabricscentral.com. In anothernon-limiting example, the material suitable for making sheet 4 mayinclude flexible PVC sheeting and fabrics manufactured/distributed byWin Plastic Extrusions of Cuyahoga Falls, Ohio, Plastic Film Corporationof Romeoville, Ill., Robeco of Garden City, N.Y., etc. In a yet furthernon-limiting example, sheet 4 may be made from coated flexible PVCsheeting such as the product marketed by Strataglass of Fort Lauderdale,Fla. under name Crystal Clear 20/20. Many of such clear and flexiblevinyl sheeting products are made for outdoor use and are hence providedwith UV inhibitors in the PVC formulation. Therefore, considering theintended exposure of the light redirecting fabric to sunlight, suchUV-treated compositions of clear and flexible PVC may be advantageouslyselected for making sheet 4.

It is noted, however, that suitable materials of sheet 4 are not limitedto plasticized PVC. Any other suitable clear and flexible thermoplasticmaterial having properties similar to PVC-P may be used for sheet 4.Particularly, such material should desirably share such properties ofPVC-P as the combination of softness and toughness, relatively easystretch, high degree of recovery, optical clarity, and resistance tosunlight. For instance, some formulations of polyethylene, polyolefinand other materials may potentially be used for making sheet 4. Thematerial should be particularly suitable for mechanical slitting in amanner such that that the walls of the slits can obtain a relativelysmooth and glossy surface when at least some types of slitting blades orrazors are used.

Certain rigid and optical clear thermoplastic polymeric materials otherthan PVC may also be modified by plasticizers to change their processingcharacteristics and potentially make a clear, strong, tough and flexiblematerial which is also suitable for slitting and obtaining a glosssurface finish within the slits. Such materials may particularly includebut are not limited to PMMA (acrylic), polycarbonate, and polystyrene.The plasticization of such rigid polymers may conventionally includemixing the respective polymeric resin with a compatible plasticizer atelevated temperature and then allowing the mixture to cool to atemperature at which the material hardens and can be further processed.The plasticizers should preferably be selected so that the plasticizedpolymer will have a glass transition temperature lower than the roomtemperature. Alternatively, the respective plasticized polymer may beheated to above the glass transition temperature for processing so thatit becomes suitable for mechanical slitting.

Sheet 4 comprises a plurality of deep and narrow slits 6 formed insurface 10. Slits 6 extend parallel to each other and also shouldpreferably extend parallel to an edge of sheet 4. The spacing betweenslits 6 can be made constant so that the plurality of such slits formsan ordered parallel array. Slits 6 thus form periodic and narrowinterruptions in otherwise smooth surface 10 and such interruptions arealternating with relatively broad spacing areas. Each slit 6 extendsthrough more than half of the thickness of sheet 4 but does not reach atthe opposing surface 12 which remains generally smooth and uninterruptedby the slits. Each slit 6 has a high aspect ratio which may be definedas the ratio between the depth of the slit and width of the slit at itsbase in surface 10. The aspect ratio of slits 6 should generally begreater than five and, more preferably, at least ten or more.

FIG. 2 shows a portion of sheet 4 of FIG. 1 in a further detail.Referring to FIG. 2, each slit 6 has a pair of non-contacting wallswhich are opposing each other and separated by a relatively smalldistance. The opposing walls of each slit are generally planar andextending generally parallel to each other and also perpendicular tosurface 10. The parallelism of the opposing walls of each slit should bewithin a few degrees, more particularly, within about six degrees. Inone embodiment, the opposing walls of each slit 6 are preferablyparallel to each other within two degrees. In one embodiment, it may bepreferred that the opposing walls of each slit 6 are parallel to eachother within one degree. Thus, each slit 6 forms a deep and narrow voidwith parallel or near-parallel walls in the material of sheet 4, whichis in contrast to V-shaped notches or grooves commonly found inprior-art light redirecting and/or waveguiding optical components. Suchvoid may be conventionally allowed to be naturally filled with theambient air.

The distance between the opposing walls of each slit 6 is substantiallyless than the distance between adjacent slits. Such distance should alsobe preferably made as small as practically possible. At the same time,the opposing walls should not be touching each other in order to preventoptical contact between them. The distance between the opposing walls,which defines the thickness of the layer of air in between, shouldpreferably be within a 5 μm to 100 μm range, and more preferably, withina 10 μm to 50 μm range. It is generally preferred that slits 6 areself-supporting when sheet 4 is deployed into a planar or nearly-planarform. To put it differently, it is preferred that sheet 4 should notgenerally require applying a constant external force to keep the slitsfrom closing.

Sheet 4 may be made in the form of a large-format sheet or wound to aroll. It may also be cut to any size or shape using any suitable fabriccutting technique. In an end-use product, sheet 4 may conventionallyhave a rectangular or square shape and a generally flat, planarconfiguration during operation as a light redirecting component.

An example of a planar rectangular configuration of sheet 4 is shown inFIG. 3 in which slits 6 extend parallel to a reference line 400 andperpendicular to a longer dimension of the sheet. Reference line 400which indicates the alignment direction of linear slits 6 in sheet 4 ishereinafter referred to as a longitudinal axis of slits 6. It should beunderstood that sheet 4 may be configured to have any other rectangularshape with any other proportions between respective dimensions. Slits 6may also be configured to extend parallel to the longer dimension of therespective rectangle or at an angle to the sides of the respectiverectangle.

Slits 6 may be conventionally formed in all of the available area ofsurface 10 so that they are uniformly distributed in parallel rowsthrough the entire span of sheet 4, as shown, for example, in FIG. 1.For some applications, however, it may be advantageous to leave someborder areas on one or more side of sheet 4 which will be free fromslits 6. A non-limiting example of such border areas provided at theopposing terminal edges of a rectangular sheet is illustrated in FIG. 3.

Slits 6 may also be formed in surface 10 according to any other suitablepattern. For example, the spacing between slits 6 may be variedaccording to a predetermined sequence or pattern. In a non-limitingexample, two or more areas of surface 10 having slits 6 may be separatedfrom each other by one or more relatively broad spacing areas to form astriped pattern of slit and non-slit areas in sheet 4.

It is noted that suitable configurations of sheet 4 are not limited tosquare or rectangular shapes and may include any other shapes such ascircular, oval, triangular, polygonal, and freeform shapes. It is alsonoted that, since sheet 4 is soft and flexible, it may also be bent toany suitable shape, wrapped around objects, etc.

A sheet of light redirecting fabric 2 cut to the appropriate size andshape may be conventionally positioned in an immediate proximity of anopening in a building façade, such as a wall or door window. It may bepositioned inside or outside of such opening, provided that at least aportion sheet 4 can be exposed to daylight. The inside location has anadvantage of better protection of the material of the fabric fromsoiling and minimizing the material degradation from sunlight andelements. On the other hand, the outside location may be advantageousfor intercepting more sunlight and potentially for more efficientdaylight harvesting.

In order to be able to harvest the direct component of natural daylight,light redirecting fabric 2 should be used in a part of the buildingfaçade which is illuminated by the direct sunlight during at least aportion of the daytime. Accordingly, unobstructed east, south and westfacades may be generally suited for using light redirecting fabric 2 forharvesting the direct sunlight. For maximum daylight capture, south,south-east and south-west facades may be more preferable.

In non-limiting example, by taking an exemplary case of the opening in abuilding façade being a vertical wall window, sheet 4 may be a suspendedin a vertical position in front of the window and may cover the entirewindow area. Such sheet 4 may be conventionally attached to the top ofthe respective window frame or to the top of the respective opening inthe wall. It should be understood, however, that sheet 4 may also bepositioned at any other location of the respective window and cover onlya portion of its area.

FIG. 4 schematically shows an example of the operation of lightredirecting fabric 2 which admits light into a building interior anddistributes light within the interior. The building interior isexemplified by a room 366 having a rectangular configuration and awindow opening 500 in its external wall 847. The incident daylight isrepresented by a ray 272 passing through window opening 500 into theroom. Ray 272 may particularly exemplify the direct sunlight or diffuseskylight which naturally propagates in a downward direction andtherefore tends to directly illuminate only the floor area in a vicinityof the window or various objects nearby.

A large-area sheet of light redirecting fabric 2 disposed in the path ofray 272 deflects such ray from its natural downward propagationdirection and redirects it onto a ceiling of room 366. There arenumerous ways of how light redirecting fabric 2 may be positioned withinor in a close proximity to such opening 500. For instance, it may befixed in a suspended position by attaching the top edge of therespective sheet 4 to wall 847 above opening 500, laminated onto awindow pane, stretched between two opposing rollers or bars, etc. Theceiling further scatters and redistributes daylight deflected by lightredirecting fabric 2 within room 366.

Redirecting daylight onto the ceiling has a number of advantages. Forinstance, considering that the incidence direction of daylight changesin a very broad angular during the daytime and seasonally, the largearea of the ceiling and its typically uniform light scatteringcharacteristics across the surface ensures that ray 272 is interceptedand properly scattered. Furthermore, since the ceiling is often paintedwhite or in relatively light colors, it may generally have a higheralbedo (reflection coefficient) than the floor or various objects in theroom interior. As a result, the light energy of ray 272 may be scatteredby the ceiling with a relatively low loss compared to scattering fromother surfaces in room 366 and thus ensure a more complete sunlightharvesting for daylighting purposes. Additionally, it may be appreciatedthat the surface of a ceiling typically has very good light diffusingproperties. Therefore, the reflection of light rays from the ceilingwill be primarily of a diffuse type which may result in a relativelyhomogeneous light distribution in the room and in a reduced glare. A yetfurther advantage of redirecting daylight to the ceiling or upperportions of the room interior is that such redirection effectivelycreates a diffuse source of daylight within the room well above the eyeheight rather than allowing the direct daylight to reflect or scatterfrom the lower surfaces and produce blinding glare. A yet furtheradvantage will be apparent when room 366 includes partitions or varioustall objects which may partially or totally obstruct daylightpenetration deep into the room interior if such daylight is notredistributed via the ceiling.

Accordingly, it will be appreciated that positioning light redirectingfabrics 2 in a close proximity or within opening 500 may provide atleast partial shading of the room interior and its occupants from thedirect sunlight while using the ceiling to convert a substantial portionof the direct beam into diffuse daylight emanated from an overheadlocation and thus enhancing the overall daylighting level and improvinglight distribution in the room interior.

FIG. 5 illustrates using a sheet of light redirecting fabric 2 toredirect and redistribute light incident onto a sloped section of a roof849 of room 366 having a different configuration compared to FIG. 4.Opening 500 of FIG. 5 may represent a conventional daylighting elementsuch as a roof window or a skylight. The sheet of light redirectingfabric 2 is positioned parallel to opening 500 and in a close proximityto it. By way of example and not limitation, the sheet of lightredirecting fabric 2 may be mounted within the frame of the respectiveroof window or skylight or otherwise attached to such frame. Lightredirecting fabric 2 may be disposed in a stretched state, laid down onan optically transmissive plate or laminated onto a planarlight-transmitting surface. Such surface may be a part of a daylightingcomponent associated with opening 500, such as, for instance, a windowpane or diffusing panel of a skylight.

In operation, referring to FIG. 5, a ray 274 exemplifying direct and/ordiffuse sunlight propagating from a zenith direction and incident intoroom 366 through opening 500 strikes light redirecting fabric 2 and isredirected onto the ceiling of room 366. Due to utilizing TIR, lightredirecting fabric 2 is capable of redirecting such ray 274 by a largebend angle which may exceed 90°. The redirected ray 274 is thenscattered and/or diffused by the ceiling which provides improveddaylighting conditions and reduced glare compared to the case when room366 is illuminated directly through opening 500 and without the aid oflight redirecting fabric 2.

FIG. 6 shows a schematic cross-section of a portion of sheet 4 andillustrates the operation of light redirecting fabric 2 in more detail.As depicted in FIG. 6, each slit 6 includes a pair of opposing parallelwalls 7 and 8 which define internal TIR optical interfaces within sheet4. Each of such optical interfaces is characterized by a stepped changein the refractive index of the medium as defined by the opticalproperties of plasticized PVC material and ambient air.

Each of walls 7 and 8 are configured to have a relatively smooth andglossy surface which is capable of reflecting light by means of totalinternal reflection (TIR) when the incidence angle of light rays ontothe surface exceeds the critical TIR angle characterizing the materialof sheet 4. Thus, each slit 6 forms a pair of internal TIR surfaceswithin sheet 4 and can be configured to redirect light at relativelyhigh bend angles using TIR. Since the transversal width of slits 6 canbe made very thin in comparison to the width of the spacing areasbetween adjacent slits, the light receiving aperture of such slits willprimarily be defined by the TIR surfaces of their walls. Accordingly,according to one aspect, the plurality of slits 6 may be viewed as aplurality of thin, two-sided TIR reflectors formed between surfaces 10and 12 of sheet 4.

When light redirecting fabric 2 is used in conjunction with a verticalwall opening, the orientation of sheet 4 should be such that slits 6extend generally parallel to the horizontal plane. In the embodimentillustrated in FIG. 6, the smooth and uninterrupted surface 12 is facingoutside and is configured as a light input surface of sheet 4. Surface10 in which slits 6 are formed is facing towards the building interiorand is configured as a light output surface of sheet 4. It is noted,however, that, according to one embodiment, sheet 4 may also bepositioned in a reverse orientation in which surface 10 is facingtowards the source of daylight (outside) and is configured for lightinput while the opposing surface 12 can be facing inside and configuredfor light output.

In operation, referring further to FIG. 6, a quasi-parallel ray bundle202 illustratively exemplifies a direct beam of sunlight striking thelight input surface 12 of sheet 4 when the sun is situated at arelatively high elevation angle with respect to the horizontal plane.More particularly, ray bundle 202 may exemplify a mid-day direct beam ofsunlight in geographical areas having relatively low latitudes.

Ray bundle 202 refracts at surface 12 and enters the optically clearmaterial of sheet 4. Since the refractive index of sheet 4 is greaterthan that of the ambient air, each ray in the ray bundle 202 undergoes apositive refraction and further propagates at a lower angle with respectto a normal 44 to surfaces 10 and 12.

As ray bundle 202 further propagates through the optically transmissivematerial of sheet 4, it strikes the surface of horizontally disposedwall 7 of one of the slits 6. Since wall 7 forms and optical interfacebetween the high-index plasticized PVC material and low-index air, itmay act as a TIR reflector for light rays striking its surface at anincidence angle greater than the critical TIR angle.

It may be shown that for the refractive indices typical for PVC material(n>1.5) and for the light path geometry illustrated in FIG. 6, the angleof incidence of ray bundle onto the surface of wall 7 will always begreater than the critical TIR angle at the PVC/air interface.Accordingly ray bundle 202 will undergo virtually lossless TIR at suchwall 7.

Referring further to FIG. 6, the operation of light redirecting fabric 2is illustrated for the case where both surfaces 7 and 8 are configuredto reflect light by means of TIR in a specular regime. The specularreflection of electromagnetic waves, and particularly light, isgenerally referred to as a mirror-like reflection from a surface inwhich a light ray or a beam of light incident from a single incomingdirection is reflected into a well-defined, single outgoing direction.Specular reflection is generally distinct from diffuse reflection inwhich an incident light ray or beam of light is reflected into a rangeof directions.

For the purpose of this invention, the term “specular reflection” shouldbe understood broadly and generally includes the type of reflection inwhich the reflected rays are distributed within a narrow angular rangefrom the specular reflection angle. Accordingly, the specularlyreflected light beam may include diffuse light rays which are deflectedfrom the “ideal” direction of the specular reflection by a relativelysmall angle.

As ray bundle 202 reflects from the surface of wall 7 by means of TIR ina specular regime, the angle of reflection will be equal to the angle ofincidence onto said surface. The spacing between adjacent slits 6 issuch that most rays in the reflected ray bundle 202 can reach the lightoutput surface 10 without being intercepted by an adjacent slit 6. Suchrays can thus undergo refraction at surface 10 and emerge from thesurface at a relatively high angle with respect to the horizontal plane.It will be appreciated that in the case of parallelism of surfaces 10and 12 and when the respective wall 7 is perpendicular to such surfaces,the propagation direction of redirected rays will generally mirror thenatural propagation direction of the beam of direct sunlight relativelyto the horizontal plane.

According to one aspect of the operation of light redirecting fabric 2,each wall 7 may act as a functional equivalent of a mirrored surfacepositioned parallel to the horizontal plane. The plurality of refractivewalls 7 distributed within sheet 4 along a vertical direction may thusbe viewed as an array of distributed reflectors inverting thepropagation direction of daylight with respect to the horizontal plane.Therefore, when such light redirecting fabric 2 is positioned in avertical orientation on a path of the direct sunlight entering a room ina building, at least a portion of such beam may be redirected onto theceiling of such room (see, e.g., the discussion in reference to FIG. 4).

The use of TIR for redirecting ray bundle 202 by sheet 4 ensures a highbend angle and directing the beam of sunlight towards the ceiling ratherthan towards the floor which may be difficult to achieve using, forexample, refractive-type light redirecting structures. Thus, in additionto the improved daylighting condition in the room, the glare andexposure of the unwanted parts of the interior to the direct sunlightmay also be reduced with the use of light redirecting fabric 2.

It is noted, however, that the spacing of slits 6 may be so selectedthat at least a portion of the high-elevation-angle beam may be allowedto continue propagate along the natural propagation direction uponexiting from surface 10. This is illustrated in FIG. 6 by the example ofa ray 82 which is intercepted by wall 8 of the above slit 6 and can beredirected differently. Since the angle of incidence of the ray 82 ontowall 8 is the same as the incidence angle of the other rays of raybundle 202 onto wall 7 and since walls 7 and 8 are generally parallel,the TIR conditions are also met for such ray at wall 8 and thereflection angle is also the same as the angle of incidence. However,since ray 82 undergoes an additional reflection from a parallel TIRreflector compared to the rest of the ray bundle 202, it continuesfollowing the original propagation direction of the respective beam.

Such splitting of the parallel beam of direct sunlight onto at least twobeams diverging from each other may be advantageously utilized tofurther improve the distribution of daylight in the building interiorand reduce glare. The angle between the main bulk of rays in ray bundle202 and ray 82 define the total beam spread of light emerging from sheet4. It will be appreciated that such beam spread may be fairly broad,particularly for the direct sunlight incident from high elevationangles. By varying the relative distance between slits 6, the ratiobetween the intensity of the beam redirected upwards and the intensityof the beam directed downwards may be changed to a desired proportionfor a given solar elevation angle thus providing additional control overdaylight distribution.

In one embodiment, the relative density of slits 6 may be madesufficiently low to enable the passage of at least some rays throughsheet 4 without any interaction with walls 7 or 8. In a non-limitingexample, illustrating a rather extreme variation of light redirectingfabric 2, slits 6 may be so sparsely populated in surface 10 that only arelatively small portion of the incident sunlight will be redirectedonto the ceiling and most the incident light may pass through sheet 4unaltered.

Conversely, in one embodiment, the relative density of slits 6 may alsobe made sufficiently high to cause multiple reflections from walls 7 and8 for at least some rays propagating through sheet 4. Thus, therespective pairs of opposing walls 7 and 8 of adjacent slits 6 may actas short-distance waveguides for such rays. The multiple interactions oflight rays with walls 7 and 8 may randomize the emergence angles fromsurface 10 thus homogenizing the light admitted into the buildinginterior. It will be appreciated that any imperfections of the surfacesof walls 7 and 8, such as waviness or surface microrelief features maycause some spreading of the initially parallel beam over an angularrange thus providing yet additional beam homogenization and furtherreducing glare.

Referring yet further to FIG. 6, a ray bundle 204 exemplifies directsunlight at a lower solar elevation angle compared to the previouslydiscussed ray bundle 202. The lower incidence angle of ray bundle 204with respect to a normal to light input surface 12 causes the respectiverays to emerge at lower angles to the horizontal plane and thuspenetrate generally deeper into the room's interior, resulting insomewhat different light distribution inside.

A ray bundle 206 exemplifies the direct sunlight at even lower solarelevation angle and may represent, for example, clear-sky outdoordaylighting conditions in winter time in the northern hemisphere or latemorning/early evening hours at low- and mid-latitude geographiclocations. The low-angle ray bundle 206 is redirected into the buildinginterior at a relatively oblique angle with respect to the horizontalplane and can penetrate considerable depth into the building. It will beappreciated that at even relative low solar elevation angles, the directbeam is still directed towards the upper portions of the buildinginterior including walls and/or the ceiling thus resulting in agenerally improved daylighting condition compared to the case whensunlight directly illuminates the floor area.

It will be appreciated that, while sheet 4 may be configured toefficiently redirect and redistribute the direct component of sunlight,it may also operate according to the same principles to admit andredistribute the diffuse (indirect) daylight into the building. Lightredirecting fabrics 2 may therefore be utilized to improve daylightinglevels in building interiors even in overcast sky conditions.

The use of TIR to redirect at least a portion of light entering into thebuilding interior through light redirecting fabric 2 allows for abroader angular distribution of light compared to refractive diffusersand can help naturally illuminate portions of the building interior thanwould not otherwise be possible. It will be appreciated that, since TIRis practically lossless, sheet 4 may have fairly high light transmissionwhich may be comparable to the transmission of a raw sheet of the samematerial.

It is noted that light redirecting fabrics 2 may be configured for asee-through appearance. The combination of the thin form factor of slits6, the transparency of the PVC-P material of sheet 4 and the parallelismand smoothness of surfaces 10 and 12 may provide for a generallyinterrupted passage of light along a direction perpendicular ornear-perpendicular to the surface of the sheet.

This is illustrated in FIG. 6 by the optical path of a light ray 980which traverses sheet 4 without appreciable attenuation or deflection.Ray 980 may particularly exemplify light scattered or otherwise emanatedby various outdoor objects that may be visible from the inside of thebuilding through the respective opening. Since sheet 4 may be configuredfor a generally unimpeded light passage at least perpendicularly to itssurface, it may have a relatively good visual transparency despite thepresence of slits 6. Accordingly, an observer 660 inside the buildingmay be able to see such outdoor objects with sufficient clarity. Asee-through configuration of light redirecting fabric 2 may beadvantageously selected for applications where the aestheticconsiderations and transparency of the respective light-admittingstructure are important.

On the other hand, since walls 7 and 8 of slits 6 are reflective, lightrays passing through sheet 4 at certain angles may undergo one or morereflections from such walls. Accordingly, images viewed through sheet 4at a sufficiently high angle from the horizontal plane may appear besomewhat distorted or otherwise may be visible with a reduced clarity.Accordingly, light redirecting fabric 2 may also be configured toprovide privacy for the occupants. In order to enhance the privacyfunction, sheet 4 may be further configured to distort the appearance ofobjects behind fabric 2 even more appreciably.

Design parameters that can be varied to control the see-through orprivacy functionality of light redirecting fabric 2 may include but arenot limited to the width and/or depth of slits 6 relatively to thethickness of sheet 4, the density of slits 6 on surface 10, angles atwhich slits 6 extend into the material and surface roughness of theslits.

When an increased level of privacy is desired, any of the surfaces 10and 12 may be patterned. Such pattern may be random or ordered and mayalso carry a decorative content such as images, silhouettes, geometricshapes or the like. Suitable patterns may be formed using conventionalmethods including but not limited to molding, calendering,micro-replication, embossing, imprinting, extrusion and the like.

In a further alternative, a pigment may be added to the material ofsheet 4 in order to alter its color or transparency. Particularly, thematerial of sheet 4 may be made translucent so that at least some imagedetails of the objects behind the sheet can be masked and/or blurred. Ina yet further alternative, sheet 4 may include or used in conjunctionwith an external translucent privacy film or screen.

For decorative purposes, any suitable image or pattern may be printed oneither surface of sheet 4. Suitable techniques may include but are notlimited to digital printing, screen printing, stencil-printing,selective dyeing and painting. The print may be opaque in which case itstotal area should preferably occupy relatively small fraction of thesurface of sheet 4 in order to not disrupt its daylighting operation.The print may also be made using transparent or semitransparent inks ordyes in which case the area of the print may be larger, up to the entirearea of sheet 4. Various shapes may also be cut out of sheet 4 in orderto provide a distinct ornamental appearance to light redirecting fabric2.

Referring yet further to FIG. 6, it will be appreciated by those skilledin the art that the reflection of light by walls 7 and 8 in a specularor near-specular regime generally requires a fairly high optical qualityof the surface. Ideally, the surface should be optically smooth andresembling the surface of optical mirrors and lenses. Conventionally,for optics and polished mirror surfaces, the calculated (or measured)values of RMS surface profile roughness do not generally exceed an upperlimit of about 100 Å (10 nanometers).

Obviously, a conventional cutting blade or razor generally cannot slitmaterials such as flexible PVC while producing the surface qualitycomparable to the optical mirrors and lenses, regardless of the bladesharpness. The conventional polishing techniques are neither availablenor practical for improving the finish of the slit walls, consideringthe softness of the PVC-P material, the extremely narrow air spacebetween walls 7 and 8 and the large number of slits 6 that may be formedin sheet 4. Therefore, the surfaces of walls 7 and 8 will rather haveappreciable roughness and highly irregular microstructure compared tothe optical-quality surfaces. Such microrelief may become readilyevident when examining the surface of an individual slit with amicroscope under 50×-1000× magnification.

Yet, it may be shown that, when special requirements are met for theslitting blade or razor and when appropriate slitting techniques areused, the surface roughness and other surface irregularities may bereduced to a level at which the surface will exhibit specular ornear-specular TIR properties sufficient for the proper operation of thelight redirecting fabric. The importance of establishing the acceptablelevels of surface roughness may be appreciated not only from the pointof view of light directing functionality and optical efficiency but alsofrom the point of view of finding the proper balance between the surfacequality and the cost of tooling for making the light redirecting fabric2. For example, on one hand, inadequate quality of the slitting blademay create light diffusing surfaces lacking sufficient specular TIRproperties of slits 6 and may thus impede their light redirectingoperation. On the other hand, setting too stringent criteria for thematerial and edge sharpness of the blade may be cost prohibitive andthus impractical for processing sheet 4 on the industrial scale.

It will be appreciated by those skilled in the art that, if a reflectivesurface is illuminated with a parallel beam of light, the reflectancemay be divided into two components, one of which arises from specularreflection and the other from diffuse reflection or scattering. It isgenerally believed that a substantially smooth reflective surface willexhibit a predominantly specular reflection while a rough surface willscatter light in a diffuse regime.

While there is no firmly established criterion of an optically smoothsurface, the so-called Rayleigh smooth surface criterion is often usedin the field of optics. The Rayleigh criterion is given as

${\left( \frac{4\; \pi \; R_{q}\cos \; \theta_{i}}{\lambda} \right)^{2}\bullet \mspace{20mu} 1},$

where R_(q) is the root mean square (RMS) roughness of the surface, λ isthe wavelength of the electromagnetic wave and θ_(i) is the angle ofincidence. The roughness parameter R_(q) commonly expresses a 2Droughness profile taken along a reference line across a portion of thesurface.

In (J. Stover, Optical Scattering, Measurement and Analysis, 2nd ed.,SPIE Press, 1995, p. 79), it is argued that the term “much less than” inthe Rayleigh criterion should mean 0.01 for electromagnetic spectrumfrom UV to the mid-IR. For 0.5 μm wavelength and for the most part ofthe expected range of incidence angles, this would translate into therequirement of RMS roughness to be well below 10 nanometers (nm). Suchsurface roughness requirement is common for conventional opticalmirrors, lenses and polished surfaces but is hardly feasible formechanical slitting the soft PVC material of sheet 4 using blades orrazors. A more conventional meaning of the term “much less than” ofabout 0.1 would yield maximum R_(q) values of around 40 nm. In (T.Vorburger, et al., Appl. Opt. 32(19), 3401-3408, 1993), the smoothsurface regime was arbitrarily defined as R_(q)/λ<0.05, which translatesinto R_(q) values of less than 25 nm for a 0.5 μm wavelength. Whilethere are clear indications in the art that the surface must meetcertain maximum roughness criteria in order to be considered opticallysmooth, the significant bias between different estimated and thevagueness of the optically smooth surface definition still leave someuncertainty regarding the acceptable roughness levels.

In order to more accurately define the requirements for surfaceroughness that can be accepted for the TIR walls of slits 6, we now turnto defining the specular regime of reflection.

It is well known that the surface rms roughness is related to the amountof specularly reflected light. The relationship governing this may befound, for example in (P. Beckmann and A. Spizzichino, The Scattering ofElectromagnetic Waves from Rough Surfaces, Artech, Norwood, Mass., 1987,Chaps. 3-5.) and can be written in the following form:

$\begin{matrix}{P_{Spec} = {P_{Tot}^{{- {(\frac{4\; \pi \; R_{q}\cos \; \theta_{i}}{\lambda})}^{2}},}}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

where P_(Spec) is the flux reflected in the specular direction andP_(Tot) is the total reflected flux. The total reflected flux is a sumof P_(Spec) and P_(Scat), the latter being the light scattered away fromthe specular direction by the surface roughness irregularities:P_(Tot)=P_(Spec)+P_(Scat).

Considering that TIR is practically lossless, Eq. 1 can be rewritten inthe following form for the case of the total internal reflection fromwalls 7 and 8 of slit 6:

$\begin{matrix}{{P_{Spec}\%} = {100\% \mspace{11mu} ^{{- {(\frac{4\; \pi \; R_{q}\cos \; \theta_{i}}{\lambda})}^{2}},}}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

where P_(Spec)% is the specular TIR reflectivity expressed as apercentage of the incident flux. Accordingly,

P _(Scat)%=100%−P _(Spec)%  (Eq. 3)

FIG. 7 schematically illustrates the effects of surface roughness on thereflection from a reflective surface 201. While it shows a ratherqualitative and general example which can be applied to many types ofsurfaces, the same type of reasoning can be applied to the case of TIRreflection from a wall of slit 6.

In FIG. 7 a, a ray 211 exemplifies a parallel beam of light incidentonto surface 201 at an incidence angle θ_(l). A ray 213 exemplifies thereflected beam which has a reflection angle θ_(r). Surface 201 of FIG. 7a exemplifies an idealized optically smooth surface which reflects ray211 entirely in a specular regime where θ_(r)=θ_(i) as a matter ofoptics.

In FIG. 7 b, surface 201 has some non-negligible roughness and thereflected light has both the specular component, exemplified by ray 213,and a diffuse component exemplified by several rays distributed over arange of reflection angles. It will be appreciated that, according toEq. 1, the relative contribution of the specular and diffuse componentsto the total reflected beam intensity will depend on the wavelength,incidence angle θ_(i) and RMS surface roughness R_(q).

Referring further to FIG. 7 b, at least some scattered light may bedistributed around ray 213 within a relatively narrow angular conehaving an angular size γ. Moreover, as it will be appreciated by thoseskilled in the art, when rms surface roughness is below a certainthreshold, a substantial part of the beam energy may be confined withinsuch narrow angular cone. The reflection distributed within a verynarrow angle is generally referred to as a specular spike and thediffuse-type reflection which has a sharply asymmetric angulardistribution with the peak intensity in the direction of the specularreflection is commonly referred to as a specular lobe.

FIG. 7 c illustrates the case of extreme, from the optical point ofview, roughness of surface 201 where all reflected light is diffuse andthere is no well defined specular component.

Obviously, the TIR walls of slits 6 formed by mechanical slittinggenerally cannot have ideally smooth surfaces operating strictly in thespecular regime of reflection of FIG. 7 a. On the other hand, the lightredirecting operation of slits 6 in a specular or near-specular TIR modegenerally precludes extreme surface roughness of the type illustrated inFIG. 7 c. Accordingly, the case illustrated in FIG. 7 b mayqualitatively describe the type of surface roughness applicable to thewalls of slits 6 more closely. Since light rays in the spectral spikegenerally maintain the propagation direction of the light beam reflectedin the “ideal” specular regime, such rays may generally be considered asbeing a part of the specular component of the reflection for the purposeof this invention. Such light rays may thus also be included in theconsideration when estimating of the acceptable levels of surfaceroughness based on the specular reflection criteria.

While the spectral lobe may have a considerably broader angular spreadcompared to the specular spike, at least a portion of its rays may stillbe confined within a relatively low angular range from the specularreflection direction. Although such rays may have a differentpropagation path compared to the rays reflected in a specula regime, therelatively low deflection angle may still permit for their redirectionin a manner similar to the specular rays. Moreover, admitting suchdiffusely reflected light into a building interior may contribute to theoverall beam spread and may thus further improve the overall daylightingconditions inside.

Accordingly, light redirecting fabric 2 may be configured so that thesurface of slits 6 includes regions having surface roughness resultingin a specular spike and a relatively narrow spectral lobe. In view ofthe above-described considerations, light redirecting fabric 2 may beconfigured with the surface roughness of slits 6 being such that atleast a substantial part of the light beam reflected from walls 7 and 8is distributed within an narrow cone having a predetermined angularvalue γ_(MAX). Angle γ_(MAX) may be defined from various considerationsincluding but not limited to the geometry of the TIR channels formed byopposing walls 7 and 8 of the adjacent slits 6, refractive index of thematerial of sheet 4, the desired angular distribution of daylight to beadmitted into the building interior, the relative proportion of lightenergy that light redirecting fabric 2 may be allowed to reject, theanticipated range of solar elevation angles, etc.

An example of defining angle γ_(MAX) based on the consideration ofminimizing the incident light rejection by sheet 4 is schematicallyillustrated in FIG. 8 which shows a portion of sheet 4 including a pairof adjacent slits 6 formed in surface 10. The surfaces of each of thewalls 7 and 8 of slits 6 include surface relief features 5. Such surfacerelief features 5 may have a random nature and may manifest themselvesas non-negligible surface roughness from the optical standpoint. Bothsurfaces 10 and 12 of FIG. 8 are smooth and generally void of anysubstantial microrelief except for the voids formed in surface 10 byslits 6.

Referring further to FIG. 8, a ray 220 exemplifies a parallel beam ofsunlight entering sheet 4 from a relatively high angular elevation. Ray220 enters sheet 4 through its light input surface 12 and strikes wall 7of the bottom slit 6. The incidence angle of ray 220 onto the horizontalsurface of the wall 7 is greater than the critical TIR angle whichcauses a total internal reflection of such ray from the surface. Surfacerelief features 5 of the respective wall 7 cause a first portion of theenergy of ray 220 to reflect in a specular or near-specular regime (asindicated by the continued path of ray 220) and causing a second portionof the ray's energy to reflect in diffuse regime over a broader angularrange. Rays 222, 224, 226, and 228 exemplify energy portions of ray 220which are deflected or scattered from the specular reflection path atvarious deflection angles.

Ray 222 is deflected by a relatively small angle and exits from thelight output surface 10 of sheet 4 slightly above the exit point of ray220 and at a different emergence angle than ray 222. Despite thedifference in the light path from the specular beam, however, ray 222still enters the building interior and is directed towards the ceilingarea, albeit at a steeper angle to the horizontal plane compared to ray220.

Ray 224, which is also deflected from the prevailing direction of ray220, emerges from surface 10 at a lower point and at a lower angle withrespect to the horizontal plane than the ray 220. Similarly, however,despite being deflected from the “ideal” direction, ray 224 also entersthe building interior and is propagated towards a direction favorablefor illumination purposes.

Accordingly, although rays 222 and 224 do not exactly follow thespecular reflection direction of ray 220, their propagation pathsgenerally conform to the intended operation of light redirecting fabric2. Additionally, such rays contribute to the angular spread of the beamadmitted into the building and thus help reduce glare and promote a moreuniform daylight distribution.

In contrast, ray 226 deviates from the path of beam 220 moreconsiderably than, for example, ray 222. Ray 226 also strikes surface 10at an angle greater than the critical angle of TIR at said surface whichcauses unwanted reflection back towards the light input surface 12. Itwill be appreciated that, unless ray 226 is eventually redirected backto surface 10 at a below-TIR incidence angle, at least a portion or evenall of its energy may be lost due to attenuation in the material ofsheet 4 or due to exiting towards the wrong side of the sheet.

Similarly, ray 228 is deflected by an even greater angle from thespecular path of ray 220. Ray 228 encounters wall 8 of the adjacent topslit 6 and strikes its surface at an incidence angle which is lower thanthe TIR angle at said surface. Ray 228 may thus traverse the respectivetop slit 6 after which it may be reflected from surface 10 in the TIRmode and directed back towards the light input surface 12. Obviously,such ray may also be lost if not redirected back again to the lightoutput surface 10 at an incidence angle that would permit ray passageinto the building interior.

Based on the principles discussed in reference to FIG. 8, angle γ_(MAX)may be selected to include at least rays 220, 222 and 224 as well as anyother rays which have sufficiently low deflection angles from the pathof ray 220. The surfaces of each wall 7 may be configured so that theirrespective surface relief features 5 disperse light within a limitedangular cone and that most of the reflected flux is distributed withinthe selected angular range ±γ_(MAX)/2. Accordingly, the relativecontribution of non-functional diffuse rays having high scatteringangles may be minimized by limiting the amplitude and/or slope angles ofsurface irregularities of slits 6 to the values which would producesufficiently narrow angular distribution of the reflected beam. In otherwords, angle γ_(MAX) may be selected to include at least the specularspike and at least a portion of the specular lobe of the reflected lightdistribution. In such a case, slits 6 having surface relief features 5and reflecting light in both specular and diffuse TIR regimes may beconfigured to redirect light in a manner similar to slits 6 configuredfor TIR in specular regime only. Since the presence of the diffusecomponent in the beam reflected from the respective wall 7 willintroduce certain angular spread to the beam of redirected sunlightemerging from the light output surface 10 of sheet 4, such redirectedbeam will be significantly softened and not as intense as the incidentone. Thus, according to one embodiment, slits 6 may be advantageouslyconfigured to provide a controlled spread of the parallel beam andfurther improve the ability for light redirecting fabric 2 toredistribute at least the direct component of sunlight over a broaderarea.

It may be appreciated that, since the amount of the specular componentand the narrowness of angular distribution of the diffuse componentstrongly depend on the surface roughness of slits 6, selecting theappropriate range of RMS surface roughness parameter R_(q) is importantfor light redirecting fabric 2 to operate properly.

Referring to Eq. 2 and FIG. 8, the incidence angle θ_(i) onto the TIRwalls of slits 6 depends mostly on the incidence angle of light onto thelight input surface 12 and the refractive index of the material of sheet4. Unplasticized PVC has the refractive index of about 1.54. Therefractive index of phthalate plasticizers typically used to makeflexible PVC sheets range between 1.48 and 1.50. Therefore, depending onthe content of plasticizers in the flexible PVC, the reflective index ofsheet 4 may conventionally be in the approximate range from 1.5 to 1.53.

Assuming the reflective index of plasticized PVC used for making sheet 4of about 1.52 and considering that walls of slits 6 are generallyperpendicular to surface 12, it can be shown that the minimum incidenceangle θ_(i) of light onto the TIR surfaces of the slits will be about49°. In other words, the TIR surfaces of slits 6 can be expected tooperate only in the incidence angle range from 49° to 90°. Since sheet 4is further expected to operate only in the visible range of theelectromagnetic spectrum corresponding to that of the daylight, theuseful range of wavelengths may also be defined. Accordingly, themaximum roughness acceptable for the walls of slits 6 can now beestimated using Eq. 2.

FIG. 9 shows calculated dependence of the specular TIR reflectivityP_(Spec)% of Eq. 2 on RMS surface roughness R_(q) at different incidenceangles θ_(i) and at 0.55 μm wavelength which is near the maximumdaylight sensitivity of the human's eye. A curve 1022 of FIG. 9corresponds to θ_(i)=49° (minimum expected incidence angle) and a curve1024 corresponds to θ_(i)=68°. It can be shown that the relativedifference of 19° between the incidence angles onto wall 7 representingcurves 1022 and 1024 corresponds to a difference in the incidence anglesonto surface 12 of approximately 46°. It will be appreciated by thoseskilled in the art that 46° is the maximum change of sun's elevationangle during the year due to the obliquity of the ecliptic.

As it is indicated by a reference threshold line 1032, a TIR surfacewhich has RMS surface roughness R_(q) of about 0.055 μm, which is1/10^(th) of the wavelength, reflects only about 50% of light in aspecular TIR regime at 49° incidence angle. It can further be seen thatat R_(q) below 0.03 μm (30 nanometers), as indicated by a referencethreshold line 1034, more than 80% of light is reflected in the specularregime and less than 20% is hereby scattered or diffused. When R_(q)approaches 0.02 μm (20 nanometers), the efficiency of specular TIRbecomes 90% or more for the entire range of the expected incidenceangles onto the surface of slits 6.

It is noted that curve 1022 represents some of the lowest expectedincidence angle θ_(i) of light rays onto the surface of slits 6 andhence some of the worst possible (from the point of view of lightreflection in a specular regime) reflection condition. Such a lowincidence angle θ_(i) may correspond, for example, to illumination ofthe light input surface of sheet 4 with a direct sunlight when sheet 4is disposed vertically and when the sun is near its zenith. At higherincidence angles θ_(i) which may correspond, for example, to lowerelevation angle of the sun, the relative amount of light reflected in aspecular regime increases, as follows from Eq. 2. This can beillustrated by comparing curve 1024 (θ_(i)=68°) to curve 1022(θ_(i)=49°) in FIG. 9.

Referring to curve 1024 of FIG. 9, it can be seen that the 50% thresholdof specular reflectivity at 68° incidence angle corresponds to RMSroughness of about 0.1 μm, as follows from Eq. 2, and that the 80%threshold corresponds to about 0.055 μm ( 1/10^(th) of the wavelength),respectively. Accordingly, it will be appreciated that the walls ofslits 6 may be configured to reflect light by means of TIR in a specularregime even if their surfaces are rough by optical standards.

The acceptable roughness may now be defined based on the estimates usingEq. 2 for the expected angular position of sheet 4 with respect to theincident sunlight. For example, when the light redirecting fabric 2employing sheet 4 in going to be used in a vertical orientation athigher latitudes (hence at lower solar elevation angles), the acceptablelevel of surface roughness may be higher than in the case of operatingit at lower latitudes (and higher sun's elevations) since the specularreflectance grows with the growth of the incidence angles θ_(i) of lightrays onto the surface of slits 6.

It is noted that, when determining the acceptable levels of surfaceroughness, the angular distribution of the diffuse component of thereflected light may also be taken into account. Particularly, eventhough the actual rms surface roughness of sits 6 may exceed a certainthreshold based on estimates obtained from Eq. 2, at least a portion ofthe diffuse light may be distributed within a relatively narrow angularcone with respect to the direction of a specular reflection. Suchportion of diffuse component may still be further directed by sheet 4into the building interior in generally the same manner as the directcomponent. Moreover configuring the TIR surfaces of slits 6 to producesuch narrowly-distributed diffuse light component may be preferred in atleast some embodiments in order to homogenize the incident daylight andreduce glare (see, e.g., discussion in reference to FIG. 8).

Accordingly, in one embodiment, it may be preferred that the RMS surfaceroughness R_(q) of at least a substantial portion of the surface ofslits 6 is in the range between 0.01 μm (10 nm) and 0.1 μm (100 nm). Inone embodiment, it may be preferred that R_(q) is generally less than0.05 μm and even more preferred that R_(q) is less than 0.03 μm.

When measuring the surface roughness parameter R_(q), it is preferredthat the sampling length is substantially greater than the wavelengthsof the daylight. Particularly, it is preferred that the sampling lengthis at least 10 μm. On the other hand, it will be appreciated thatmacroscopic irregularities of the surface may have a certain impact onthe measured roughness but may not necessarily influence thr surfacereflectance in a specular regime. In this view, selecting too longsampling lengths, in comparison to the size of the characteristicmicrostructures causing diffuse reflection, may potentially result inoverestimating R_(q) for the purpose of calculating the specularreflectance due to the increased role of such macro irregularities.Accordingly, especially when macro-irregularities are present on thesurface of the walls of slits 6, it may be preferred that surfaceroughness parameter R_(q) is measured over a sampling length whichminimizes the impact of such non-indicative macro-relief and yetprovides statistically meaningful results. Particularly, it may bepreferred that the sampling lengths for estimating the characteristicR_(q) of the surface of slits 6 are selected to be within the range of20 μm to 100 μm.

It is noted that the above criteria of surface roughness generally applyto the measure of the surface micro-irregularities in the plane ofreflection. It will be appreciated that the roughness measured in anorthogonal plane may have less impact on the angular distribution of thereflected light in the plane of reflection, particularly for the casewhen the surface irregularities are greater than the wavelength of theincident light. This is illustrated in FIG. 10 which depicts an TIRsurface 231 and a reflection of a ray 251 from such surface. Surface 231may exemplify a portion of the surface of one of the walls of anindividual linear slit 6 and ray 251 may exemplify a light rayreflecting from such wall in a reflection plane 240 which isperpendicular to the longitudinal axis 400 of such linear slit 6. A line260 indicates the intersection line of reflection plane 240 with surface231. A plane 241 which is perpendicular to surface 231 and parallel tothe longitudinal axis 400 of the respective slit 6 crosses surface 231at line 261.

Referring further to FIG. 10, the roughness of surface 231 in the planeof reflection 240 may be sufficiently low to enable TIR in a specularregime. At the same time, the roughness of surface 231 in the orthogonalplane 241 may be much greater, as illustrated by a plurality of parallelundulations 237 which affect the profile of line 261 but have no impacton the profile of line 260. In other words, the two-dimensional profilealong line 261 is much more irregular than the 2D profile along line260, which can be reflected in a significant difference in R_(q) whenmeasured along the respective lines.

Undulations 237 may exemplify surface irregularities formed by aslitting blade or razor used to produce slits 6 in the material of sheet4. While the amplitude of such irregularities may be such that the RMSroughness measured along line 261 exceeds the maximum preferredroughness values, it should be understood that the roughness measuredalong line 260 is more relevant for determining the surface quality forthe intended operation of the respective slit 6. Accordingly, in oneembodiment, it may be preferred that the surface profile roughness R_(q)does not exceed the prescribed values along a line which is eitherperpendicular to the longitudinal axis 400 of the respective slit 6 oris disposed at a relatively high angle with respect to such axis.

It will be appreciated that the minimum requirements for the surfacesmoothness of slits 6, albeit significantly more relaxed compared toconventional optics, put fairly strict requirements on the hardware andthe proccess used to produce such slits. Limiting the RMS surfaceroughness to only a few tens of nanometers requires a relatively highquality of the slitting blades or razors that may be used to producesuch slits. Particularly, the cutting edge of the slitting blade shouldpreferably be extremely sharp, finely polished or honed and essentiallyburr-free. The blade or at least its cutting edge area should alsopreferably be made from a stiff and hard material such as hardenedcarbon steel, tungsten carbide, diamond, certain ceramics, and the like.Hard coatings such as amorphous diamond, diamond-like carbon-(DLC)material, nitrides, carbides, oxides or ceramics may also be used toimprove the hardness and strength of the blade's edge. In someimplementations, the cutting edge of the slitting blade may also beprovided with an outer layer of low-friction material, such asPolytetrafluoroethylene (PTFE), in order to reduce the cutting frictionof the blade with the material of sheet 4. If the tip of the slittingblade is rounded (in a transversal cross-section), the radius of thecurvature of the tip in said cross-section should preferably be on asub-micron scale. It may be further preferred that such radius ofcurvature is less than 50 nanometers.

FIG. 11( a) through FIG. 11( d) schematically illustrate an embodimentof a method of making light redirecting fabric 2, more particularlyillustrating steps of forming the parallel array of slits 6 in sheet 4and separating the opposite walls of the slits from each other.

Referring to FIG. 11( a), which depicts a step of such method, slits 6are initially formed in surface 10 of sheet 4 using a relatively thinand sharp blade 9. Blade 9 penetrates relatively deep into the softPVC-P material of sheet 4 and makes the cut by wedging the material outto the sides on its way. The elasticity and easy deformability of thesoft PVC material permit for a relatively easy cut formation. Thematerial deforms elastically and relatively easily yields under thecutting pressure, leaving a clean cut without chipping, crazing orirregular tearing.

As discussed above, blade 9 should preferably be extremely sharp (with asub-micron curvature radius of the tip), burr-free and made from a hardmaterial. The walls of blade 9 at least near the cutting edge shouldalso be highly polished to a very low level of surface roughness. In oneembodiment, the RMS surface roughness of blade 9 near the cutting edgeshould preferably be below 100 nanometers and, even more preferably,below 50 nanometers.

Since such slitting does not generally result in material removal fromthe slit areas and since plasticized PVC generally has a high degree ofrecovery, the opposing walls of the newly formed slits 6 may tend toclose upon each other after blade 9 is removed. Closing slits 6 mayresult in the opposing walls of such slits coming into an intimateoptical contact with each other thus frustrating the total internalreflection which is required for light redirecting operation of lightredirecting fabric 2. In other words, even though the structure of sheet4 may be significantly altered in the areas where slits 6 are formed(left of FIG. 11( a)), such sheet may still generally lack thesufficient light redirecting properties and optically behave somewhatsimilar to the uncut portion of the sheet (right of FIG. 11( a)).

Referring to FIG. 11( b), which shows the portion of sheet 4 of FIG. 11(a) after the slitting process is complete, slits 6 remain closed withthe opposing walls of each slit 6 disposed in an intimate contact witheach other. The opposing walls of each slit need to be separated inorder to provide the basic light-directing functionality of lightredirecting fabric 2. The main purpose of such separation is formingnarrow, self-supporting voids which can be naturally filled with theambient air thus enabling TIR interfaces for light redirection.Additionally, the permanent separation of slits 6 soon after slitting isdone may generally prevent forming an unwanted bond between the slitwalls during a prolonged storage. This may be particularly important forthe flexible PVC materials which are known for a slow release ofplasticizers from the surface (the so-called leaching). Such leachingmay cause migration of plasticizers into slits 6 and furtherstrengthening the bond and optical contact between their walls which maypotentially complicate the subsequent wall separation and formingquality TIR surfaces.

Referring to FIG. 11( c), the wall separation of slits 6 may employ astep of stretching sheet 4 along a direction which is perpendicular tothe longitudinal axis of the slits, according to an embodiment of themethod of making light redirecting fabric 2. This step may involveapplying a load to sheet 4 along directions 582 and 584 which causesstretch elongation of the sheet along such directions.

It will be appreciated that, since the effective thickness of sheet 4 inthe areas below slits 6 is substantially less than in the spacing areas,the elongation of sheet 4 will be highly disproportional along thestretch direction. The portions of sheet 4 having smaller thickness willgenerally experience greater stress levels and greater elongation thanthe thicker portions of the sheet. Accordingly, the elongation of sheet4 will occur primarily due to the stretch deformation of the material inthe areas of slits 6 and particularly in the areas below the tips ofeach slit 6. As a result, the distance between walls 7 and 8 of eachslit 6 will increase in accordance with such elongation thus allowingthe ambient air to fill the respective void.

In order to make the separation of walls 7 and 8 permanent, sheet 4 maybe annealed at an elevated temperature. Such thermal annealing may atleast partially relieve the stresses related to stretch elongation andtherefore make at least a portion of the elongation irreversible. Suchstep is further illustrated in FIG. 11( c) which also shows a heatsource 550 positioned underneath sheet 4 in proximity of surface 12.Heat source 550 is configured to heat at least the bottom portion ofsheet 4 where the low-thickness regions below the tips of slits 6 arelocated. The elevated temperature usually causes a reduction in thethermoplastic material's elasticity and its ability to recover from thestretch deformation. The temperature of the material of sheet 4 and theduration of exposure of sheet 4 to heat source 550 should be sufficientto reduce the internal stresses in such regions to a point where atleast a part of the stretch deformation becomes irreversible. It isnoted that the combined effect of elevated temperature and stretchdeformation of sheet 4 in this step may also cause some plasticdeformation of the material of sheet 4 in the areas of slits 6 which.Depending on the temperature of the respective portions of sheet 4, theamount of applied load as well as the length of processing time, theflow of the material of sheet 4 in the plastic deformation regime undersuch load may contribute to or even be the main factor in theirreversible separation of walls 7 and 8 from each other.

FIG. 11( d) schematically depicts the final step of making lightredirecting fabric 2 in which the tension load and heat source areremoved and sheet 4 generally returns to its original shape whileleaving deep and narrow channels or voids in surface 10 corresponding toslits 6. The permanent elongation of the material of sheet 4 in theareas just below each slit 6 prevents walls 7 and 8 of each slit fromcontacting each other and thus preventing the slits to close. Since thevoids in the material of sheet 4 become self-supporting, a minimumdistance and a minimum air gap between walls 7 and 8 required for TIRoperation may now be maintained without an external load.

Considering that the processing step of FIG. 11( c) causes at least someirreversible elongation of the material of sheet 4 in the areas of slits6, the residual length of sheet 4 after the load is removed in the stepof FIG. 11( d) may become slightly greater than the original length ofthe sheet. However, since slits 6 occupy only a small fraction of thesurface of sheet 4 and since the sheet material in the wide spacingareas between slits 6 may remain generally intact, the relativeirreversible elongation of the entire sheet 4 can be approximately equalto the sum of the final widths of all slits 6. In one embodiment, it maybe preferred that the relative irreversible elongation of sheet 4 doesnot exceed 10% of the original sheet length. In one embodiment, it maybe preferred that such elongation is less than 5% of the originallength. It is also important that stretching of sheet 4 in the plasticdeformation mode should be controlled in order to prevent overextendingthe sheet, avoid material tearing as well as prevent the formation oftoo wide or distorted slits 6.

FIG. 12 depicts the above-discussed process of slit wall separation in afurther detail by showing an example of irreversible widening of anindividual slit 6 by forming an individual self-supporting void in sheet4 in the area of such slit. A portion of sheet 4 of FIG. 12 a includinga single slit 6 is initially stretched along directions 582 and 584which are perpendicular to the prevailing plane of slit 6. Sinceflexible PVC is inherently elastic, sheet 4 may be stretched primarilyin the elastic mode by applying a suitable load. It will be appreciatedthat the elastic elongation of sheet 4 will be generally proportional tothe applied load.

The term “elastic elongation” in relation to a stretch-tensioned sheetmaterial is directed to mean a temporary elongation of the sheet as aresult of pull stress in a particular direction. Once the stress isremoved, the sheet may return to its original shape and length alongsuch direction. The elastic elongation is also directed to mean that theapplied stress is less than the elastic limit of the sheet material.

An area 23 immediately underneath slit 6 is characterized by asignificantly reduced thickness of sheet 4 compared to the areas of thesheet between the slits. The reduced thickness translates into a reducedarea of a cross-section of the material in a plane perpendicular to theforce direction. Therefore, at a given stretch load, the stress appliedto the material of sheet 4 in area 23 and the elongation of the sheetmaterial in such area will be much greater than in the surroundingareas. At a certain combination of heat applied to area 23 by heatsource 550 and the tension force applied to sheet 4 along directions 582and 584, the material in area 23 may begin to flow in the plasticdeformation mode along such directions, thus causing irreversiblewidening of slit 6.

FIG. 13 depicts a generalized schematic stress-strain curve 1010 of athermoplastic material which can also be applied to describing thebehavior of sheet 4 under stretch loads. The strain due to elongation isusually expressed in terms of L/l, where L is the original length of thematerial and l is the elongation length. The stress 6 is usuallymeasured in terms of F/A, where F is the applied forth and A is the areaof a cross-section of the material in a plane perpendicular to the forcedirection. σ_(Y) indicates a yield strength of the material and σ_(TS)indicates its tensile strength at which point the material may break.Various formulations of plasticized PVC allow for 40% to 300% elongationuntil the material can break.

The stress-strain curve of thermoplastic materials including flexiblePVC typically features a plastic region, represented by a portion ofcurve 1010 between threshold lines 1012 and 1014, where the materialdraws and extends in a plastic deformation mode. “Plastic deformation”is the deformation that remains after a load is removed from thematerial. In contrast to the elastic deformation, plastic deformation ispermanent and generally non-recoverable.

Accordingly, referring back to FIG. 12( a), heating the bottom portionof sheet 4 in the presence of stretch load along directions 582 and 584may cause plastic deformation in area 23 thus making the materialelongation in such area at least partially irreversible. This may beachieved by applying a pull force sufficient to cause the materialstrain and stress above the respective ε_(Y) and σ_(Y) for the effectivethickness of sheet 4 material in area 23.

At the same time, it is preferred that the pull force is substantiallyless than the force needed to exceed the yield stress σ_(Y) of therespective plasticized PVC material at a nominal sheet thickness inorder to avoid irreversible deformation in the spacing areas betweenslits 6. In other words, the elongation of sheet 4 in space areasbetween slits 6 should remain substantially elastic. A point 462 oncurve 1010 of FIG. 13 schematically indicates an operational point whichcorresponds to operational values of strain ε_(OP) and stress σ_(OP) ofthe uncut portions of sheet 4 so that σ_(OP)□σ_(Y) and ε_(OP)□ε_(Y).Since most elastic materials have a characteristic linear elastic regionon the stress-strain curve where stress is roughly proportional to thestrain (Hooke's Law), point 462 may be advantageously selected to bewithin such region.

It is noted that the appreciable viscous flow of material in areas 23 isnot necessarily required to permanently separate walls 7 and 8 of slits6. As discussed above in reference to FIG. 11( c), the elevatedtemperature in areas 23 may simply provide stress relaxation (annealing)for the existing elastic deformation and still result in irreversiblechanges of the material in such areas. The stress relaxation is directedto mean the progressive loss of stress (load) over time under constantstrain (deformation), usually at elevated temperatures.

In other words, the elevated temperature may be used to convert theelastic elongation of the material in areas 23 into a plastic elongationwithout exceeding the respective ε_(Y) and σ_(Y) values in such areas.Accordingly, in one embodiment, sheet 4 may be stretched in the elasticmode under a constant load and then heated to a predeterminedtemperature and annealed for a period of time to complete the slitwidening process. In one embodiment, the annealing temperatures may bein the range between 60° C. and 150° C.

In a different implementation of the method of making the lightredirecting fabric 2, the step of permanent separation of the walls ofslits 6 may employ additional stretching of sheet 4 along directions 582and 564. Particularly, instead of using heat source 550 to lower theyield strength σ_(Y) of the material of sheet 4 at a given strain, sheet4 may be further stretched beyond its yield strength σ_(Y) in areas 23at a constant temperature. The stretch load applied to sheet 4 shouldstill be less than the breaking point σ_(TS) (see FIG. 13) of thematerial in areas 23, however. Obviously, this may also result inplastic deformation in the “thin” areas 23 while the rest of the area ofsheet 4 may remain unaffected due to the much greater thickness of thematerial between the slits.

In one embodiment, the viscoelastic behavior of plasticized PVC at largedeformations may also be exploited. For instance, it was found thatflexible PVC sheets may undergo a permanent deformation withoutappreciable material flow when strained past 100% or so. Accordingly,stretching the material of sheet 4 in areas 23 beyond such threshold mayalso be used for enabling plastic deformation in said areas andpermanent widening of slits 6.

The amount of the desired widening of slits 6 depends on the requireddistance between walls 7 and 8 of the slits and may be controlled by thetension force, the amount of heat applied to sheet 4 and the duration ofstretching in the plastic deformation or annealing mode. Since each area23 may only need to be heated for a brief period of time at a constanttension, a series of areas 23 may be heated and/or annealed in acontinuous process as the respective portions of sheet 4 are movedacross the heat source 550. Such operation maybe performed during theslitting process or as a separate step following the slit formation.

The temperature regime and the tension of sheet 4 may be so selectedthat the planar shapes of surfaces 10 and 12 remain mostly unaltered andthe material deformation is localized in relatively compact areas 23. Itwill be appreciated that when the elongation of sheet 4 in areas 23 issufficiently small, only small portions of walls 7 and 8 near the tip ofthe respective slit 6 will experience distortions by the plasticdeformation while the rest of their surfaces may substantially retainthe planar shape and perpendicular orientation with respect to thesurfaces 10 and 12 of sheet 4.

Referring to FIG. 12( b), the removal of stretch load from sheet 4 mayresult in some recovery of the material in area 23 and partial recoveryof the shape of slit 6 due to the material elasticity. However, since atleast some of the stretch elongation in area 23 was made irreversible inthe step of FIG. 12( a), walls 7 and 8 remain separated from each otherby at least a small distance thus providing TIR interfaced for lightredirecting operation and allowing for a thin layer of ambient air toform between such walls.

A width W_(S) of an individual slit 6 may be defined as the distancebetween walls 7 and 8 measured at a particular depth of the slit in atransversal cross-section (see FIG. 12( b)). It is generally preferredthat W_(S) is made fairly small along the entire active area of the slit6 so that walls 7 and 8 can be nearly perpendicular to surfaces 10 and12 and also parallel to each other. It may also be preferred that W_(S)does not exceed a certain maximum value W_(s). In one embodiment, W_(S)_(—) _(MAX) may be selected to be less than 10% of the thickness ofsheet 4. According to one embodiment, it is also preferred that theaverage value of width W_(S) is substantially less than the thickness ofthe blade used to produce the respective slit 6. In a non-limitingexample, the slitting blade may be 0.3 mm thick and the width W_(S) ofslits 6 of the finished light redirecting fabric may be in the rangebetween 0.01 to 0.1 mm.

It is noted that the amount of relative stretching of sheet 4 in theelastic mode may have a very minimum impact on the width W_(S) of slits6, if any, due to the high recovery of at least some formulations ofplasticized PVC. Therefore, slit width W_(S) will be defined by themanner in which the step of permanent slit separation is performed.

The minimum practical value of width W_(S) of slits 6 depends on severalfactors. One such factor is the requirement of maintaining the TIRinterfaces between the material of sheet 4 and air at walls 7 and 8 ofslits 6 and preventing the contact of the walls with each other.Additionally, it may be preferred that a minimum air gap between thewalls 7 and 8 is maintained to prevent the frustrated total internalreflection which may result from the penetration of an evanescent wavefrom the material of sheet 4 into air. It will be appreciated by thoseskilled in the art of optics that TIR may be frustrated due to thepassage of the evanescent wave through an extremely thin air gap even ifthe adjacent TIR surfaces are not physically contacting each other. Thepenetration depth of the evanescent wave depends on the wavelength andthe angle of incidence and is usually of the order of the wavelength.

An additional factor which may influence the selection of the desiredminimum width of slits 6 is the possible unevenness and/ornon-parallelism of walls 7 and 8. For example, if either one or bothwalls exhibit macroscopic surface irregularities, the minimum widthshould accommodate the height of such surface irregularities.

Yet additional factors requiring consideration for determining theminimum desired width of slits 6 may include the environment associatedwith the subsequent use of the formed sheet 4. For example, the thermalexpansion or contraction of the material of sheet 4 or flexing the sheetas a whole may potentially cause distortions which can bring walls 7 and8 in contact with each other during normal use if the width of slits 6is inadequate.

Accordingly, in one embodiment, it may be preferred that the minimumwidth of each slit 6 is at least 5 μm to 10 μm. However, it should beunderstood that this estimate is not prescriptive and should not beconstrued as limiting this invention in any way. The actual width W_(S)may also be influenced by a number of other factors, particularly thoserelated to the manufacturing process, tolerances, material properties ofsheet 4, etc., and may generally exceed such minimum width. As apractical consideration, according to one embodiment, the minimum widthof the active portion of each slits 6 may be preferred to be generallygreater than 10 μm.

FIG. 14 illustrates an exemplary implementation of a method of makinglight redirecting fabric 2. It also shows an embodiment of an apparatusfor slitting sheet 4 and separating the opposing walls of the respectiveslits 6.

Referring to FIG. 14( a), elongated rectangular sheet 4 of opticallyclear, flexible PVC is formed into a closed loop by connecting andseaming its two opposing edges together. The edges may be connectedtogether by any suitable means, including but not limited to atear-resistant adhesive tape, thermal welding, vinyl cement or othertype adhesive. The loop of sheet 4 is stretched between two opposingparallel rollers 552 and 554.

A next step illustrated in FIG. 14( b) includes elastic stretchtensioning of sheet 4 along a direction perpendicular to the intendedslitting direction in order to build sufficient internal stress in suchdirection. Due to the elasticity of the PVC-P material, sheet 4 maynormally undergo elastic elongation along the direction of the appliedstress. The amount of stretch load which should be applied to sheet 4depends on many factors which may include enabling the additionalelongation of sheet 4 in areas 23 of slits 6 to be formed, preventingthe slippage of the sheet material in place during the cut operation,removing excess sag of sheet 4, reducing the friction between a cuttingblade and the sheet, reducing the required slitting pressure that shouldbe applied to the blade, improving wall separation of slits 6 and otherconsiderations.

Various methods may be used for such tensioning. One method may includeelongation of sheet 4 by moving rollers 552 and 554 further apart fromeach other, as indicated by directions 582 and 584. For instance, roller554 may be fixed in a stationary position and the tension can beprovided by moving roller 552 away from roller 554. It will beappreciated that, when at least some initial tension is applied, thetotal elongation of sheet 4 will be generally proportional to theincrease in the distance between the rollers and the stress on sheet 4will be defined by the pull force applied to roller 552.

Other methods of elastic stretch tensioning of sheet 4 may includeemploying an additional tensioning roller or a section of a web in whichdifferent tension can be created in said section compared to the rest ofthe web. It may be preferred that the method used for tensioning sheet 4provides means for keeping the tension approximately constant regardlessof the additional elongation of sheet 4 during the subsequentprocessing.

Referring further to FIG. 14( b) blade 9 of a rotary type is provided tomake parallel cuts in the material of sheet 4. Such rotary blade 9 ispositioned above roller 554 with its plane parallel to the axis of theroller and is made movable across the entire width of sheet 4. A linearsliding rail system with a stepped drive (not shown) may be provided toeffectuate the reciprocal motion of blade 9 across sheet 4. The cuttingprocess starts by moving blade 9 parallel to the roller 554 across thematerial of sheet 4, as indicated by a direction 579. The movement ofblade 9 along direction 579 continues until a full-length individuallinear slit 6 is formed in sheet 4.

The penetration depth of blade 9 into sheet 4 is primarily defined bythe desired depth of the slits 6 to be formed in the sheet. According toone embodiment, the slitting depth should be more than 50% and less than95% of the thickness of the sheet material. Among different factors thatmay determine the desired cut depth can be the consideration of theresidual stiffness and tear resistance of sheet 4. By way of example andnot limitation and considering that the self-supporting large-areasheets of vinyl films in the thickness range of 2 to 10 mil (1 mil is0.001 of an inch) are quite common, the depth of the cut may be up to75-90% of the thickness of the sheet material if sheet 4 is at least 0.5mm thick, so that at least 0.05 mm to 0.25 mm of the material remainsavailable for holding the sheet together.

Roller 554 may be optionally provided with a heating element so that theheat can be transferred to the slitting area of sheet 4 in order toadditionally soften the material in the respective area and aid theslitting process. A liquid lubricating agent may be optionally sprayedonto sheet or otherwise applied to surface 10 of sheet 4 or to thecutting edge of blade 9 in order to reduce the friction between theblade and the sheet material and to further promote the slittingprocess. The stretch tension applied along directions 582 and 584 mayhelp pull the material apart in the area of the cut thus reducing thefriction with blade 9 and further assisting the slitting process.

Referring to FIG. 14( c), roller 554 may be provided with a steppeddrive (not shown) which rotates the roller by a small predeterminedangle after each slit 6 is formed so that the top surface of sheet 4makes periodic feed movements along a direction 570. The drive of roller554 can be synchronized with the linear drive that moves blade 9 so thatthe reciprocal slitting operation of the blade is alternating with theshort feed increments of sheet 4. Accordingly, referring further to FIG.14( c), such process can form an array of parallel slits 6 in thesurface of sheet 4 at specific intervals. The process may continue untilall of the available surface 10 of sheet 4 is processed.

The distance between slits 6 can be controlled by the feed incrementsprovided by the respective drive of roller 554. In one embodiment, suchfeed increments can be made constant thus forming an array of slits 6having a constant pitch. In one embodiment, the drive or roller 554 maybe configured to provide variable feed increments thus varying thespacing between slits 6.

Referring yet further to FIG. 14( c), heat source 550 is positioned inthe space between rollers 552 and 554 and configured to heat a portionof sheet 4 in which slits 6 are already formed. Heat source 550 mayinclude any suitable heating elements or heat sources that can be usedfor heating a specific portion of sheet 4 by means of radiant heattransfer, thermal conduction or convection. It may also include an airblower unit to deliver such heat to the sheet surface. Alternatively,heat source 550 may include a heat transfer roller which can be pressedagainst sheet 4 and heat a narrow portion of the sheet to the requiredtemperature.

In one embodiment, heat source 550 may be particularly configured toheat the material of sheet 4 to a temperature at which the elasticitylimit at a given strain in the thinned areas of sheet (such as areas 23of FIG. 12( a)) is exceeded in order to cause the material of sheet 4 toyield in a plastic deformation mode in such areas. As explained inreference to FIG. 11( c) and FIG. 12( a), due to the lower thickness ofthe material in the areas of slits 6 compared to the spacing areas, theplastic deformation of sheet 4 will primarily occur in the immediatevicinity of the tips of the slits, thus permanently widening the voidsformed by the slits. When the stretch load is removed, slits 6 will thusretain at least a part of their deformation of the stretched state. Inother words, the uneven thickness of sheet 4 due to the presence ofslits 6 creates uneven stress and plastic elongation of the sheet in thethinnest areas. The plastic elongation in the areas of slits 6 isconsiderably greater than that in the uncut areas of the sheet whichmakes it possible to widen the slits by a predetermined amount withoutthe undue distortion of the main bulk of the sheet material and withoutappreciable distortion of its surfaces.

In one embodiment, heat source 550 may be configured to providetemperature sufficient for at least partial stress relaxation in areas23 of sheet 4 and making at least a part of the elastic elongation insuch areas permanent without an appreciable material flow. In oneembodiment, heat source 550 and the feed rate of sheet 4 may beconfigured to provide the temperature and heating time for therespective portions of sheet 4 so as to cause both plastic deformationand thermal annealing of sheet 4 in the respective areas 23.

Accordingly, upon the completion of the process of forming slits 6 andheating the areas of such slits under a constant tension, an array ofself-supporting voids in surface 10 of the sheet may be formed. Each ofsuch voids formed by the respective slit 6 may thus be naturally filledwith the ambient air and provide a pair of TIR interfaces suitable forlight redirecting operation of sheet 4. The connected ends of the loopof sheet 4 may now be separated so that the sheet can take a generallyplanar form or can be wound into a roll for storage.

Example 1

A rectangular sheet of 40-gage (around 1 mm-thick) clear flexible vinyl(PVC) was formed into a loop and elastically stretched between twospaced apart, parallel rollers at a constant tension. A straight-edgerotary blade made from tungsten tool steel and manufactured by OLFA wasused to produce an array of parallel slits in the flexible PVC sheet.The blade had the diameter of 28 mm and the thickness of around 0.3 mm(OLFA product number RB28). The blade was positioned perpendicular tothe stretch direction on a linear guiding rail system and the slittingaction was performed by a reciprocal movement of the blade through theentire width of the vinyl sheet using a linear actuator. The feed of theflexible vinyl sheet was performed by rotating the feed roller in smallangular increments using a stepped drive coupled to the roller's axis.The depth of blade penetration into the sheet material was 0.75 mm whichproduced linear slits with nearly vertical walls and the depth of around0.75 mm. The pitch or the slits was made constant with spacing betweenadjacent slits being around 0.5 mm.

The subsequent stretching and annealing the vinyl sheet in plasticdeformation mode at an elevated temperature has permanently opened theslits by forming narrow, air-filled gaps between the walls of each slit.The slits remained open after the stretch load and the source of heatremoved were removed.

The processed sheet was suspended in a window opening inside a room of asouth-facing building façade and exposed to a beam of direct sunlight.The sheet has produced an extended bright spot on the ceiling of theroom while casting a well defined shadow on the floor.

The processed sheet was then separated along linear slits in severalareas selected for sampling which exposed the TIR surfaces of respectiveslits. The surface roughness of the slit walls was measured using Zeta200 optical microscope/surface profiler from Nanoscience Instruments.The RMS surface roughness R_(q) parameter was measured along a profiledirection which is perpendicular to the longitudinal axis of therespective slit. The measurements were made at 50 μm and 100 μm samplinglengths at different locations of the slit surface. The RMS roughnessparameter R_(q) was found to be varying generally between 0.011 μm (11nm) and 0.030 μm (30 nm) from sample to sample.

Example 2

A rectangular sheet of 40-gage clear flexible vinyl was prepared andprocessed in much the same manner as in Example 1 except that anothercutting blade was used. The new blade was a 28-mm rotary tungstensteel/carbon steel blade manufactured by Kai Industries (part #C-280BL).Such blades yielded the RMS surface profile roughness parameter R_(q) inthe range between 0.012 μm (12 nm) and 0.025 μm (25 nm), correspondingto the calculated specular TIR reflectivity of the slit surfaces of atleast 85% for the entire targeted range of incidence angles θ_(i) (from49° to 90°). When exposed to direct sunlight, the processed sheet hasproduced a similar bright spot on the ceiling by redirecting therespective beam form the original propagation direction.

FIG. 15 depicts an exemplary surface profile measured for a wall of oneof the slits made in Example 2 above. The X coordinate shows a locallinear coordinate on the surface of the slit wall measured along theperpendicular to the longitudinal axis of the respective slit. The Ycoordinate shows the vertical step-heights of the profile as measured bythe profiling instrument. As it may be seen, the surface of the slitwall exhibited some measurable surface roughness in the form of randomrelief characterized by alternating peaks and valleys of variousheights. However, as indicated by the measured RMS roughness parameterR_(q), the scale of such roughness being in low tenths of nanometersgenerally favored the specular TIR from the surface at least at thetargeted incidence angles.

FIG. 16 schematically illustrates a variation in the apparatus andmethod of making light redirecting fabric 2. In FIG. 16, an array ofparallel slits 6 in surface 10 of sheet 4 is formed by making acontinuous spiral cut in the material of the sheet. Sheet 4 is formedinto a loop and stretched between opposing rollers 552 and 554 similarlyto FIG. 14( a). The stretch tension should be sufficient to maintain agood traction between the rollers and sheet 4 and preventing sheetslippage. Rotary blade 9 is positioned above roller 554 so that theplanar surface of the blade is perpendicular to the axis of roller 554and parallel to the stretch direction. Blade 9 is also attached to alinear motion mechanism so that it can be moved along direction 579which is parallel to the axis of roller 554.

Either one of the rollers 552 or 554 can be provided with a drivingmotor which should rotate the respective roller at a constant angularrate. Such rotation of the driving roller causes a constant-rate motionof the surface of sheet 4 with respect to blade 9. For slittingoperation, blade 9 is initially positioned at one side of sheet 4 andlowered into the material of the sheet without touching the surface ofroller 554, according to the desired depth of slits 6 and according tothe desired remaining thickness of the material in the area of the slit.After blade 9 is lowered to the appropriate slitting depth, it is movedalong direction 579 at a slow, constant speed. As blade 9 reaches theopposing side of sheet 4, a continuous spiral slit will be formedcovering the entire surface 10 of the sheet. It will be appreciatedthat, when the loop of sheet 4 is subsequently cut along direction 579,the continuous spiral slit will be transformed into a linear array ofparallel slits 6.

The subsequent steps of making the light redirecting fabric 2 mayinclude stretching sheet 4 in a direction perpendicular to slits 6 andapplying heat to permanently separate the opposing walls each slit fromone another by means of plastic deformation in areas 23 and/or bythermal annealing, as explained above in reference to FIG. 11 and FIG.12.

The method of making the light redirecting fabric 2 is not limited tousing a single blade. For example, multiple straight or rotary bladesmay be assembled into a pack which can cut multiple slits 6 in a singlepass. Additionally, the method of making the light redirecting fabric isnot limited to processing individual large-format rectangular sheets andmay be adapted to process a continuous sheet 4 from a web on aroll-to-roll basis.

FIG. 17 shows an exemplary apparatus for processing sheet 4 in whichslits 6 are formed using a dense pack of ten rotary blades 9 assembledon a common axis 480. Sheet 4 is supplied on a roll 572 and is fedthrough support rollers 576 and 578 onto a receiving roll 574.

The supply roll 572 may be provided with a controlled tensioningmechanism, such as an unwind brake commonly used for rolls tensioning inthe film processing industry. Such mechanism may be configured toprovide a constant slip tension so that sheet 4 can be unwound from theroll under a constant stretch tension.

Roll 574 may be provided with a torque drive mechanism configured torewind sheet 4 from roll 572 to roll 574 in predefined, steppedincrements. A support member 770 is positioned at a small distance fromsheet 4 parallel to its surface on the opposite side from the pack ofblades 9. Its function is to provide a solid guiding surface forslitting and prevent excessive sheet sagging under blade pressure.

The pack of blades 9 and support member 770 form a slitting block of theapparatus shown in FIG. 17. Heat source 550 is positioned on the exitside from the slitting block and is configured to heat a portion of theslit area of sheet 4 to a temperature sufficient to convert at leastsome of the elastic elongation of the sheet into plastic (irreversible)elongation.

In operation, the pack of blades 9 makes a reciprocal motionperpendicularly to the feed direction so that each blade 9 penetratesthrough surface 10 into sheet 4 and forms a deep and narrow slit 6 whichresults in creating a parallel array of ten slits in a single pass.While being slit, sheet 4 may be pressed by blades 9 against supportmember 770 to ensure an even slitting pressure and a constant depth ofthe cut.

After each pass of blades 9 through sheet 4, roll 574 rewinds the sheetby a predetermined increment to expose the next uncut portion of thesheet. It may be appreciated that, the use of multiple blades may beadvantageous compared to using a single blade in terms of speed of sheet4 processing and may be preferred in the mass production environment.

Blades 9 may be spaced apart from each other by a constant or variabledistance which may be selected depending on various factors includingthe properties of the material of sheet 4, the thickness of each bladeand other considerations. Such blade spacing will, in turn, affect theincrements at which sheet 4 needs to be rewound after each pass. This isillustrated in FIG. 18 which shows forming an array of slits 9 usingthree blades that are spaced apart by approximately three nominalspacing distanced of the slits.

Referring to FIG. 18 a, blades 9 initially form three slits in surface10 with the spacing areas being three times the intended width. Sincethe slitting process does not generally involve material removal fromsheet 4, a more tight packing of blades 9 may prevent or make itdifficult to penetrate into the sheet material and perform the cleancuts, depending on the thickness of the blades, the thickness of sheet 4and the slit spacing. Therefore, the use of such sparsely spaced bladesmay have an advantage over the case where the blades are disposed veryclose to each other according to the spacing between slits 6.

As illustrated in FIG. 18 b, after cutting the first series of slits 6is complete, the pack of blades 9 is moved transversally with respect tothe longitudinal axis of the slits by a distance approximating theintended pitch of slits 6. As explained above, this relative motion maybe performed, for example, by an incremental feed of sheet 4 in responseto the incremental rotation of roll 574 of FIG. 17. In areverse-direction slitting motion, blades 9 cut three more slits 9 inmanner similar to FIG. 18 a. The process of feeding sheet 4 and thereciprocal motion of blades 9 is then repeated again to form three moreslits 9 in the remaining spacing areas (FIG. 18 c).

FIG. 18 d shows the results of the slitting operation of the sparselypopulated pack of blades 9. At the following slitting step, sheet 4 maybe fed by a larger increment which will expose the next uncut area ofits surface 10.

FIG. 18 e shows sheet 4 with the fully formed slits 6 after thepreviously discussed thermal annealing or plastic stretch-elongation ofthe sheet. The controlled plastic deformation at the tips of slits 6results in a permanent separation of the opposing walls 7 and 8 of theslits and filling the respective openings with ambient air, thusproviding permanent TIR interfaces for light redirecting operation.

It is noted that the number of blades 9 which may be included in thearray for simultaneous making of a series of slits 6 in sheet 4 can beany other than those shown in FIG. 17 and FIG. 18. In one embodiment,the pack of blades may have as many blades 9 as needed to cover asignificant portion or the entire area of sheet 4 and to slit therespective sheet in just a few passes or even a single pass. Multipleblades 9 may also be arranged in any other suitable configuration. Forexample, blades 9 may be positioned at different angles with respect tosurface 10 of sheet 4. In another example, blades may also be disposedin a staggered arrangement. Blades 9 may also be spread over multiplecutting blocks and each block may be configured to slit a different areaof sheet 4 in a simultaneous or sequential operation.

It is further noted that suitable means for stretching sheet 4 along adirection perpendicular to linear slits 6 is not limited to rollers orrolls such as those shown in FIG. 14 and FIG. 17 and may include otherconventional elements, such as clamps, actuators, air cylinders, and thelike. Sheet 4 is also not limited to be formed into a loop forstretching or to be supplied from a roll. For instance, a pair ofclamping bars may be provided. Sheet 4 may clamped to such bars at itsedges and stretched between the clamping bars. Sheet 4 may be eithermoved relatively to a stationary cutting head comprising one or moreblades 9 or, alternatively, the sheet may be positioned stationary andthe slitting operation may be performed by a moving head.

The foregoing embodiments are described upon the case where surface 12,which is generally smooth and uninterrupted by the slits, is configuredfor light input and the opposing surface 10 comprising slits 6 isconfigured for light output. However, this invention is not limited tothis and can be applied to the case when surface 10 is configured forlight input and surface 12 is configured for light output. It will beappreciated that, when both surfaces 10 and 12 are generally smooth,slits 6 are perpendicular to the prevailing plane of sheet 4 and theopposing walls of each slit are parallel to each other, the lightredirecting operation of the light redirecting fabric may besubstantially the same regardless of which surface is facing inside oroutside of the building interior. In other words, light redirectingfabric 2 may be configured to be bifacial and fully operational in bothorientations.

FIG. 19 shows an embodiment of light redirecting fabric 2 which issimilar to that of FIG. 6 but in which the slit surface 10 is configuredfor light input and the opposing uninterrupted surface 12 is configuredfor light output. Additionally, walls 7 and 8 of FIG. 19 are configuredto have a more substantial surface roughness compared to the caseillustrated in FIG. 6 which provides the respective slits with somebeam-spreading function.

The operation of the embodiment of FIG. 19 may be better understoodreferring to the illustration of light reflection from an opticallyrough surface of individual slit 6 of FIG. 8. Similarly, while surfaceroughness of walls 7 and 8 in FIG. 19 may be non-negligible, thecharacter of such roughness may be controlled so that each wall of slits6 reflects light in near-specular regime with a very narrow angularspread of the reflected beam.

Accordingly, parallel light beams represented by ray bundles 202, 204and 205 in FIG. 19 are redirected and admitted into the buildinginterior with some notable divergence at least in the reflection plane.Such divergence may be advantageous for making the daylight distributionmore uniform and/or propagating the daylight into deeper areas of thebuilding interior. The respective surface relief features 5 (not shownin FIG. 19) may also be configured to cause beam divergence in a planeperpendicular to the plane of reflection thus even further reducingglare and improving lighting uniformity.

It will be appreciated that the surface relief features 5 which may bepresent on the surfaces of slits 6 and configured for beam spreadingwill not necessarily affect the transparency and see-through appearanceof sheet 4 along a surface normal. Therefore, light redirecting fabric 2employing light-diffusing slits 6 may still be used in a manner whichallows for viewing objects behind it, as indicated by the path of ray980 in FIG. 19.

FIG. 20 shows an embodiment similar to FIG. 6 except that the lightoutput surface 10 is patterned with light diffusing microstructure. Themicrostructured pattern of surface 10 may be formed by an array orrandom or ordered surface relief features 18 which can have any2-dimensional or 3-dimentional geometry configured for dispersing orscattering a parallel beam of light by means of refracting its raystowards different directions. Suitable optical micrustructures mayinclude but are not limited to prism arrays, arrays of prisms, prismaticgrooves, lens arrays, engineered surfaces, various surface relief typescommonly referred to as “frosted-glass”, “prismatic”, “sanded”,“pebble”, “ice”, “matte”, “microprism”, “microlens”, and the like.Alternatively or in addition to that, surface 10 may have any decorativeor ornamental microstructured features such as, for example, those foundin window privacy films or screens.

Any conventional means used to pattern the surface of plastic sheets orfilms may be used to form the textured surface 10. Surface patterning iscommonly incorporated in the processes of making sheets of films of aflexible material and may involve techniques such as extrusion, casting,molding, imprinting, calendaring, etching and the like. It is preferred,particularly for the case when sheet 4 is made from clear plasticizedPVC, that the microstructured surface 10 is formed at the time of makingsheet 4 so that slits 6 can be cut in the already patterned surface.

In operation, referring further to FIG. 20, ray bundles 202, 204 and206, representing parallel beams of direct sunlight at different solarelevation angles and entering sheet 4 through light input surface 12,are redirected by slits 6 using TIR and are further dispersed over abroad angular range by surface relief features 18 of surface 10. Atleast a large fraction of each of the divergent beams emerging fromsurface 10 is generally directed towards the upper portions of thebuilding interior, such as the ceiling or tops of the walls and may thusbe further redistributed due to secondary reflections or scattering.Since daylight emerging from sheet 4 is at least partially diffused andpropagates in the form of a divergent beam, it may reach different areasof the building interior including locations at a considerable distancefrom the respective window while producing a relatively low level ofglare.

Due to the presence of surface texture in sheet 4, the transparency ofsuch embodiment of light redirecting fabric 2 may be substantiallyreduced compared to the case when sheet 4 has generally smooth surfaces10 and 12. This is illustrated by example of the path of ray 980 in FIG.20. It is seen that such ray 980 initially propagating from a distantoutdoor object along a perpendicular to surfaces 12 and 10 is deflectedfrom its initial propagation path by microstructured surface 10 and doesnot therefore reach the observer's eye 660 disposed on the line ofsight. Accordingly, it will be appreciated that such implementation oflight redirecting fabric 2 may be advantageously used for theapplications requiring enhanced levels of privacy.

Either one of the surfaces 10 and 12 may be textured in order to enhancethe light diffusing properties of light redirecting fabric 2 and/orenhance its privacy functions. However, when such textured surface isconfigured for a broad-angle scattering, sheet 4 should preferably beused in an orientation where the textured surface is facing away fromthe source of daylight and towards the building interior. Suchpreference may be understood considering that the too high scatteringangles may suppress TIR at slits 6 or otherwise cause unwanted TIR atthe light output surface of sheet 4 (see, e.g., the discussion inreference to rays 226 and 228 of FIG. 8).

It is noted that, when one of the surfaces of sheet 4 is textured andsuch textured surface is properly facing away from the source ofdaylight, the respective surface microstructure may be used to enhancethe light throughput of light redirecting fabric 2, especially in thecase where the surfaces of slits 6 have some excessive roughness orwaviness.

This is further illustrated in FIG. 21 which shows a portion of sheet 4and incident ray paths similar to FIG. 8 except that the light outputsurface 10 includes surface relief features 18 of FIG. 20. The addedrelief of surface 10 causes scattering of rays 220, 222 and 224 inrandom directions within a certain angular range. It will be appreciatedby those skilled in the art that such surface relief may also suppressthe total reflection for at least some rays striking surface 10 atangles with respect to a surface normal greater than the critical TIRangle. For instance, referring to FIG. 21, rays 226 and 228 are nowtransmitted and scattered by surface 10 in a forward direction ratherthan being reflected by means of TIR as in the case of FIG. 8. As aresult, rays 226 and 228 are also directed into the building interiordespite their high deflection angles resulting from the surfaceroughness of the respective slit 6.

Thus, as the example of FIG. 21 illustrates, the microstructured surface10 may be configured to at least partially compensate the negativeeffect of the excess roughness that may be present in walls 7 and/or 8of slits 6. It may be appreciated that providing a suitable micro-relieffor the light output surface of sheet 4 can be particularly useful forenhancing the light transmittance of light redirecting fabric 2 atrelatively high incidence angles. For example, referring back to FIG. 8it may be shown that, at high incidence angles of light rays onto sheet4, even relatively small deviations of rays from the “ideal” reflectionpath of ray 220 upon interacting with wall 7 may result ingreater-than-TIR incidence angles onto surface 10. Therefore, accordingto one embodiment, the light output surface of sheet 4 may includesurface relief features 18 configured to suppress TIR at such surfaceand broaden the acceptance angle of light redirecting fabric 2.

Referring further to FIG. 21, the optical structure formed by sheet 4and a plurality of slits 6 also generally illustrates a method forredirecting light using a sheet of optically transmissive material. Suchmethod includes (a) propagating the light in sheet 4 in response to theoptical transmission; (b) reflecting light by a total internalreflection of at a plurality of slits 6 distributed within the materialof the sheet; and (c) extracting the reflected light from light outputsurface 10. The surfaces of slits 6 may particularly include surfaceirregularities which are considered significant by optical standards.Particularly, walls 7 and/or 8 may include areas characterized by theRMS surface profile roughness parameter in the range from 10 nanometersto 100 nanometers at a sampling length between 20 and 100 micrometers.

Light redirecting fabric 2 may comprise any number of additionalinternal or external layers that can have various functions. Forinstance, referring to FIG. 22, a layer 40 of an optically transmissivematerial may be provided on at least one surface of sheet 4. By way ofexample and not limitation, such layer 40 may be applied to surface 10and used to provide protective encapsulation of slits 6 and preventdust, dirt and/or moisture accumulating in slits 6. In a furtherexample, layer 40 may be formed by an optically clear adhesive which canbe used for laminating sheet 4 to other surfaces.

FIG. 22 also shows an external layer 42 that may be attached to a lightoutput surface of sheet 4. Such layer may include surface relieffeatures 18 configured for diffusing light emerging from sheet 4 and/orbroadening the acceptance angle of light redirecting fabric 2. Layer 42and its surface relief features 18 may be formed separately from sheet 4and then laminated to its surface. Any layer in a multilayered structureof sheet 4 may also be provided with UV- or IR-blocking properties.Alternatively, or in addition to that, either layer may be provided withcolor filtering properties or tint.

Suitable configurations of sheet 4 are not limited to the perpendicularorientation of slits 6 with respect to the sheet surface. In oneembodiment, one or more slits 6 may be formed at an angle with respectto a surface normal.

FIG. 23 depicts a portion of vertically positioned sheet 4 in whichsurface 12 is configured for light input and the opposing surface 10 isconfigured for light output. Slits 6 are formed in surface 10 at aconstant non-zero slope angle δ with respect to a normal 45 to saidsurface. As further illustrated in FIG. 23, such sloped slits 6 may beformed by blade 9 which is tilted at such angle δ with respect to normal45 during the slitting process. As a result of a non-zero tilt angle ofblade 9, each of the planar walls 7 and 8 will form a certain dihedralangle Ω with the prevailing plane of surface 10 which will be less than90°. More particularly, due to the general parallelism of walls 7 and 8,dihedral angle Ω of each of the walls will generally be complementary toslope angle δ, that is Ω=90°−δ.

Surface 10 of FIG. 23 further includes random surface relief features 18which may be configured for diffusing the transmitted light and/orsuppressing TIR at such surface.

In operation, ray bundles 203 and 205 exemplifying a quasi-parallel beamof the direct sunlight strike surface 12 which refracts the respectivelight rays and admits them into the optically transmissive material ofsheet 4. The spacing between slits 6 and the dihedral angle Ω of eachslit are such that all rays of ray bundles 203 and 205 are interceptedby the respective walls 7. At least a substantial portion of the surfaceof walls 7 has the RMS surface roughness generally below 50 nm, and morepreferably, below 30 nm. As a result, at least 85% of light incidentonto walls 7 may be reflected in a specular regime by means of TIR.

Since each slit 6 is disposed at an angle with respect to the horizontalplane, the deflection angle of each ray is such that the respectivelight beams are reflected and redirected at greater angles with respectto such plane. Surface relief features 18 further scatter or diffuse thenear-parallel beams of ray bundles 203 and 205 and result in surface 10emitting generally divergent light beams towards the ceiling area.Features 18 may be configured to disperse light over a relatively broadangle so that the individual light beams reflected by respective slits 6may superimpose on each other resulting in a uniform, diffuse beam ofdaylight distributed over the entire area of the building's interior.

FIG. 25 schematically depicts an exemplary method of making sloped slits6 in sheet 4. In one embodiment, such method may be incorporated as astep in the method of making the light redirecting fabric 2. In FIG. 25,sheet 4 is stretched over a roller 555 which rotates in small periodicangular increments alternating with the reciprocal movements of blade 9in a direction parallel to the axis of roller 555. The suitable tilt ofblade 9 with respect to a normal to surface 10 is obtained by offsettingthe point of entry of blade 9 into the material of sheet 4 by apredetermined distance from a radial position with respect to roller555. As the slitting process ensues, slits 6 having a constant slopewith respect to a surface normal are formed.

FIG. 24 shows an embodiment similar to FIG. 23 except that the slopeangle δ of slits 6 is variable. Such variable slope may be obtained byvarying the tilt angle of blade(s) 9 with respect to the surface normal.For instance, referring to FIG. 25, by varying the offset of blade 9from the symmetrical radial position, the slope of slits 6 with respectto the surface normal may also be varied.

Referring yet further to FIG. 25, it may also be appreciated that, whenthe diameter of roller 555 is sufficiently small, slitting sheet 4 overits cylindrical surface with an offset may also produce slightly curvedwalls of slits 6. Accordingly, in one embodiment, the TIR surfaces ofslits 6 may be configured to have a curvilinear shape. For example, suchslits 6 having curvilinear walls may be employed for light diffusingpurposes.

In one embodiment, slope angle δ of slits 6 may be chosen randomly in apredetermined angular range. When sheet 4 is illuminated by a parallelbeam of light, such as direct sunlight, the random orientation of slits6 may result in a random orientation of the redirected light rays thusalso providing additional light diffusing function.

FIG. 26 shows an embodiment of light redirecting fabric 2 having analternative arrangement of parallel slits 6 in surface 10 compared tothe case when slits 6 are a formed in continuous parallel rows extendingfrom one edge of sheet 4 to the other. Referring to FIG. 26, each linearslit 6 has a length which is considerably shorter than the width ofsheet 4 along axis 400. The plurality of slits 6 is arranged instaggered rows and columns where each row is shifted relatively to theadjacent rows. In each row, the adjacent slits 6 are separated by aspacing area of the uncut bulk sheet material. Such arrangement of slits6 may be useful, for example, for the case when the remaining thicknessof the material under the slits would be insufficient to support theintegrity of sheet 4 if each slit is continuously extending end-to-end.

It will also be appreciated that such staggered arrangement of slits 6may also allow for each slit to extend through the entire thickness ofthe material of sheet 4 without losing the integrity of the sheet as awhole. The thickness of the material of sheet 4, the length of each slit6 and the spacing distance between the slits may be selected so that thematerial in the spacing areas can support the shape of sheet 4.

It is noted that the arrangement of slits 6 of FIG. 26 also illustratesan exemplary implementation of light redirecting fabric 2 in which anuncut border area is preserved along a perimeter of sheet 4. Leavinguncut areas at one or more edges of sheet 4 may be advantageous, forexample for enhancing the tear resistance of light redirecting fabric 2.

Suitable implementations of light redirecting fabric 2 are not limitedto arranging slits 6 into a single array in which all such slits extendparallel to one particular reference line. Slits 6 may also be arrangedin two or more arrays which may be adjacent to each other, spaced apart,overlapping or disposed in any angular position with respect of eachother. In one embodiment, different arrays of slits 6 may be formed inthe same surface of sheet 4. In one embodiment, different arrays ofslits 6 may be formed in the opposing surface of sheet 4.

FIG. 27 shows an embodiment in which slits 6 are formed in two differentarrays and said arrays are crossed at a right angle to each other.Referring to FIG. 27, a first array of slits 6 extends parallel toreference line 400 and a second array of slits 6 extends parallel toanother reference line 401 which is perpendicular to line 400.

Since the walls of slits 6 of the respective arrays are separated fromeach other by a thin layer of air in both dimensions and can beconfigured to reflect light in a specular or near-specular TIR regime,the intersecting slits may thus form two-dimensional light-channelingcells. Such light-channeling cells may be configured to split anoff-axis parallel beam into two or more beams propagating into opposingdirections with respect to a normal to the light input surface of sheet4.

This is illustrated in FIG. 28 in which a light channeling cell 30 isformed, in a cross-section, by wall 7 of one slit 6 and wall 8 of anadjacent slit 6. An exemplary off-axis ray 82 passes through sheet 4without interacting with any of the walls of the respective lightchanneling cell 30 and therefore maintains its original propagationdirection upon exiting from surface 12. A different off-axis ray 84enters sheet 4 from the same direction but strikes the entrance apertureof cell 30 at a different location than ray 82. Ray 84 undergoesreflection from wall 7 by means of TIR and exits from sheet 4 towards adifferent direction which mirrors the direction of ray 82 with respectto normal 45.

Rays 82 and 84 of FIG. 28 may particularly exemplify a parallel beam ofdirect sunlight at a low or moderate solar elevation angle when sheet 4is disposed in a horizontal orientation. Although rays 82 and 84 areincident from a single direction, the difference in their propagationpaths results in such rays being deflected symmetrically away fromnormal 45 resulting in a highly divergent light beam. As a matter ofoptics, the angle between the propagation directions of divergent rays82 and 84 can be about twice the incidence angle of such rays ontosurface 10. Accordingly, the resulting divergence angle of a parallelbeam passing onto such cell 30 strongly depends on the angle that theincident beam makes with respect to normal 45. Particularly, when theincidence angle is 45° or more, the full angle of the divergent beam mayexceed 90° even in the absence of any appreciable scattering at TIRwalls of slits 6 or surfaces 10 and 12 of sheet 4.

It will be appreciated that the relative dimensions of cells 30 may becontrolled by varying the density and/or depth of slits 6. Cells 30 maybe particularly configured to have a relatively high aspect ratio bymaking slits 6 with the appropriately high density and depth, in whichcase at least some rays may even undergo multiple reflections from theopposing walls 7 and 8 of cells 30. Such kaleidoscopic reflections mayresult in a further randomization of the paths of off-axis rays thusimproving the uniformity of beam splitting.

It should be noted that the operation of individual cells 30 is notlimited to reflecting light from the opposing parallel TIR walls 7 and 8in a single plane. For example, FIG. 29 illustrates a ray path throughan individual cell 30 where the light ray enters surface 10 from arandom off-axis direction. As it is seen, such ray can be sequentiallyreflected from one or more of the four walls of light-channeling cell 30thus obtaining a random emergence angle in both angular dimensions.

It will be appreciated that, when cell 30 is configured accordingly andexposed to a beam of light which can be represented by a large number ofparallel off-axis rays evenly distributed over the entrance aperture ofthe cell, such rays may randomly mix within cell 30 and emerge fromsurface 12 at random orientations. Accordingly, each cell 30 may beconfigured to operate similarly to a short-length kaleidoscopic lightpipe and provide light diffusing functions by dispersing a parallel beamof light into different directions spanning across a broad angularrange. Particularly, daylilghting fabric 2 employing perpendiculararrays of slits 6 may be used to improve the diffusion and distributionof daylight incident from different angles due to the diurnal and/orseasonal motion of the sun.

Light redirecting fabric 2 may also be configured to combine the beamsplitting or kaleidoscopic function of cells 30 with light diffusingfunctions of various surfaces of sheet 4 and/or slits 6. For example,walls 7 and 8 may be configured to include light-diffusing surfacerelief features 5 of FIG. 8. Alternatively, or in addition to that, thedesignated light output surface of sheet 4 may be configured to includesurface relief features 18 of FIG. 20.

By way of example and not limitation, such configuration of lightredirecting fabric 2 may be incorporated into a skylight in a roof orceiling of a building. This is illustrated in reference to FIG. 30 andFIG. 31 in which a ray 280 represents a beam of direct sunlight passingthrough horizontal opening 500 in the ceiling of room 366 at an angle toa vertical direction. Such opening 500 may represent the exit apertureof a skylight configured to illuminate room 366 with daylight.

In order to be able to compare and contrast the present invention withtypical light diffusing optical elements used in daylighting systems,FIG. 30 is shown to illustrate the operation of conventional lightdiffusing panel 378 positioned on the path of ray 280. Light diffusingpanel 378 typically employs a molded sheet of optically clear ortranslucent polymeric material which includes light diffusingmicrostrustures such as a lens arrays, prism arrays, and the like.

Due to the refractive nature of light redirection by microstructuredsurfaces, the bend angle provided by light diffusing panel 378 islimited to 30-45° or so. As a result, the full angular width of thediffuse beam is usually less than 90°. This is illustrated by a fan ofrays 700 which represents the diffuse beam produced by panel 378 whenilluminated by ray 280. Additionally, the intensity of such diffuse beamusually peaks along the continuation direction of the incident directbeam (corresponding to zero deflection angle) and fades considerably atlarger deflection angles. The diffuse beam of panel 378 also tends topropagate generally towards the same direction as ray 280. All thisresults in the fan of rays 700 of FIG. 30 having a sharply asymmetricpropagation direction and an angular width which is insufficient todirectly illuminate the entire room. More particularly, such fan of rays700 directly illuminates only a portion of room 366 located on the sideopposite to the incidence direction of ray 280. Thus, the respectiveportion of room 366 may become over-illuminated and even include “hot”spots of extreme light intensity while the illumination level of theopposing side of the room may remain too low.

In contrast, referring now to FIG. 31, light redirecting fabric 2 may beconfigured to diffuse and spread the direct beam of ray 280 over a muchbroader angular cone and provide more even illumination of the entireroom interior. A sheet of such light redirecting fabric 32 may besimilarly positioned in a horizontal orientation on the path of ray 280as the panel 378 of FIG. 30. Since the sheet of light redirecting fabric2 can be quite flexible and prone to sagging if not supported properly,it may be, for example, stretched between suitable frame members (notshown), laid down on a light transmitting grid or laminated onto atransparent plate or panel. Slits 6 of sheet 4 may be arranged in twoperpendicular arrays so as to form a plurality of light channeling cells30, such as those discussed in reference to FIG. 28 and FIG. 29. Lightredirecting fabric 2 of FIG. 31 may additionally include light diffusingmeans. Such light diffusing means may particularly employlight-diffusing surface relief features 5 of the walls of the respectiveslits 6 and/or light-diffusing pattern of surface relief features 18 ofthe light output surface of sheet 4.

In operation, daylilghting fabric 2 of FIG. 31 distributes the lightenergy of ray 280 over a broad angular range and illuminates room 366with a relatively uniform diffuse beam. In one embodiment, lightredirecting fabric 2 may be configured to produce a generally symmetricbeam with respect to a normal to its surface. In one embodiment, lightredirecting fabric 2 may also be configured so that the respective fanof rays 700 has the angular span of more than 90° for at least for someelevation angles of the sun so that the opposing portions of room 366may be adequately illuminated. Accordingly, it will be appreciated thatsuch configuration of light redirecting fabric 2 of FIG. 31 may providedaylighting conditions within room 366 which are far superior to thoseprovided by conventional light diffusing panel of FIG. 30.

FIG. 32 depicts a further example of employing light redirecting fabric2, according to one embodiment of the present invention. Referring toFIG. 32, rectangular sheet 4 of light redirecting fabric 2 is laminatedonto a surface of a window pane 300. Window pane 300 may represent, forexample, a planar glass pane in a window of a building façade. Moregenerally, it may also represent the surface of an opticallytransmissive glazing found in daylighting elements of a building such asa wall window, door window, roof window, and the like.

Sheet 4 is made from a soft, flexible and optically transmissivematerial, preferably clear plasticized PVC with the thickness between0.5 mm to 1.5 mm. Surface 12 of sheet 4 should preferably be made smoothand suitable for lamination onto another surface with a good physicaland optical contact. Particularly, surface 12 may be configured to makesheet 4 attachable to a smooth glass surface by means of lamination. Forthis purpose, surface 12 may be optionally calendered using is a seriesof hard pressure rollers to enhance its smoothness. In order to furtherenhance the lamination efficiency and/or adhesion to glass, surface 12may be specially treated for high surface energy or static clingproperties. In order to further enhance the adhesion to a glass surface,a layer of optically-clear adhesive may be provided between surface 12and the respective surface of pane 300. A low-tack adhesive may beselected to provide for a durable lamination and yet relatively easyremovability of sheet 4 from pane 300. A moderate- to high-tack adhesivemay be used if more permanent adhesion of sheet 4 to pane 300 isrequired by the specific application.

The size of sheet 4 may be selected to cover only a portion or theentire surface of pane 300. When the size of sheet 4 is selected tocover only o a portion of the area of pane 300, such sheet may belaminated onto any suitable part of the window pane. Variousconsiderations for positioning sheet 4 may include but are not limitedto the exposure of the respective part of pane 300 to the directsunlight, the desirability of view obstruction, privacy, ease of access,various environmental factors, etc. In one embodiment, sheet 4 may bepositioned on the surface of window pane 300 so that it is generallyabove the eye height of the building occupants.

Surface 12 of sheet 4 in FIG. 32 is configured for light input andsurface 10 configured for light output from the sheet. The orientationof sheet 4 is such that slits 6 formed in surface 10 extend generallyparallel to the horizontal plane. Slits 6 are configured to redirect andredistribute sunlight incident onto the respective portion of pane 300at least from relatively high solar elevation angles. Particularly,slits 6 are configured to redirect at least a portion of such sunlightonto a ceiling of the respective building interior according to theprinciples of above-discussed embodiments.

In one embodiment, light redirecting fabric 2 of FIG. 32 may beconfigured with both surfaces 10 and 12 being smooth and free of anysurface relief features. In one embodiment, suitable micro-relief may beprovided on surface 10 in the form of surface relief features 18 inorder to enhance the light diffusing function of sheet 4, as furtherillustrated in FIG. 33.

Referring to FIG. 33, vertically-oriented glass window pane 300 has anouter broad-area surface 302 and an opposing inner broad-area surface304 which is parallel to surface 302. Surface 302 is facing the outsideof the building and is exposed to the incident sunlight, while surface304 is facing the building interior. Sheet 4 of light redirecting fabric2 is laminated onto the inner surface 304 so that its surface 12 forms agood physical and optical contact with surface 304. Surface 12 of sheet4 is configured for light input and is made smooth and preferablycalendered in order to promote such contact with surface 304 and reduceforming air bubbles or wrinkles. A layer of optically clear adhesive maybe used to promote long-lasting adhesion.

Referring further to FIG. 33, the opposing light output surface 10includes a plurality of deep and narrow slits 6 formed perpendicularlyto the prevailing plane of sheet 4 and arranged in a parallel array.Sheet 4 is oriented in such a way that the longitudinal axis of slits 6is aligned parallel to the horizontal plane. It will be appreciatedthat, at such orientation, the respective planes of walls 7 and 8 ofslits 6 will also be disposed generally parallel to the horizontalplane. Each slit 6 has a relatively smooth surface and is configured toinclude a thin layer of air between the respective pair of walls 7 and 8thus enabling wall reflectivity by means of TIR. The surfaces of atleast walls 7 are sufficiently smooth to reflect light by means of TIRin a specular or near-specular regime. The surfaces of the opposingwalls 8 are also preferably smooth and configured for TIR in a specularor near-specular regime. However, since the illustrated orientation ofsheet 4 provides for daylight redirection mostly by walls 7, walls 8 mayhave more uneven or even rough surface without impairing thefunctionality of light redirecting fabric 2. Surface 10 of sheet 4further includes a plurality of light diffusing surface relief features18 which are configured to extract light from sheet 4 and disperse suchlight within a building interior in the form of a diffuse beam.

In operation, referring yet further to FIG. 33, ray bundles 202, 204 and206 illustratively represent direct beams of sunlight striking the outersurface 302 of window pane 300 at different incidence angles. Windowpane 300 may be conventionally made of glass which has a relatively hightransmittance of sunlight. When sheet 4 is made of plasticized PVC, itsrefractive index may be is closely approximating that of glass, in whichcase the Fresnel reflection at the optical interface between pane 300and sheet 4 will be virtually eliminated resulting in a high efficiencyof light passage from window pane 300 to sheet 4. TIR at walls 7 ofslits 6 is practically lossless and the transmittance of clear PVC at amillimeter thickness can be quite high at the targeted wavelengths.Furthermore, surface relief features 18 of surface 10 may be configuredto include a microlens pattern which has a relatively high opticalthroughput in a broad range of incidence angles. Accordingly, thedaylighting device formed by sheet 4 of light redirecting fabric 2laminated onto window pane 300 may have a relatively high lighttransmittance and optical efficiency. Therefore, each of the ray bundlesillustrated in FIG. 33 may be transmitted through sheet 4 with minimumlosses and redirected towards the ceiling of the respective buildinginterior.

It is noted that the lamination of sheet 4 onto window pane 300 may beimplemented in various alternative ways. For example, in one embodiment,sheet 4 may be applied to the outside surface 302 of pane 300. In oneembodiment, sheet 4 may be laminated onto pane 300 with its surface 10facing towards the pane. It will be appreciated that in such a case,slits 6 made in surface 10 can be fully or partially encapsulated by thecontact of surfaces 304 and 10 and may at least partially be isolatedfrom the environment. Such configuration may be advantageously selected,for example, in a situation where the contamination of slits 6 with dustand/or unwanted residues or moisture can be a concern.

Furthermore, sheet 4 may be laminated onto pane 300 as a secondarylayer. For instance, a window film may be laminated onto a surface ofpane 300 first and then sheet 4 may be laminated onto such window film.In one embodiment, the window film may be additionally configured forsuitable light filtering properties, such as blocking the infra-red orultra-violet rays, etc. In one embodiment, such window film may also beconfigured to provide a certain tint to the sunlight admitted into thebuilding interior. It will be appreciated that many window panes includesome form of coatings, such as those found in low-emissivity windows.Accordingly, sheet 4 may be laminated onto such coated windows eitherdirectly or by means of any number of intermediate layers whilemaintaining the same basic configuration and operation principles.

It is also noted that the embodiment shown in FIG. 33 is not limited toany particular type of windows in a building and can be applied to manyforms of building glazing used for admitting daylight into the interior.Such types of glazing may include but are not limited to single-panewindows, dual-pane and multi-pane windows, glass doors,light-transmitting window coverings, transparent walls or ceilings, andthe like. In dual-pane or multi-pane glazing, sheet 4 may also beincorporated between the panes in order to provide a better protectionof the light redirecting fabric from the environment. It is furthernoted that one or more sheets light redirecting fabric 2 may belaminated to a planar or curved surface of a glazing element disposed inany suitable orientation with respect to the horizontal plane, includingparallel, perpendicular or sloped orientations. For surfaces havingtwo-dimensional curvature, light redirecting fabric 2 may also be cutinto a suitable template that will conform to the shape of the surface,stretched over such shape or applied in the form of a mosaic of multiplesmaller sheets, patches or strips.

Further details of the structure and operation of the light redirectingfabric 2 and the method and apparatus for making the same, as shown inthe drawing figures, as well as their possible variations will beapparent from the foregoing description of preferred embodiments.Although the description above contains many details, these should notbe construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. Therefore, it will be appreciated that the scope ofthe present invention fully encompasses other embodiments which maybecome obvious to those skilled in the art, and that the scope of thepresent invention is accordingly to be limited by nothing other than theappended claims, in which reference to an element in the singular is notintended to mean “one and only one” unless explicitly so stated, butrather “one or more.” All structural, chemical, and functionalequivalents to the elements of the above-described preferred embodimentthat are known to those of ordinary skill in the art are expresslyincorporated herein by reference and are intended to be encompassed bythe present claims. Moreover, it is not necessary for a device or methodto address each and every problem sought to be solved by the presentinvention, for it to be encompassed by the present claims. Furthermore,no element, component, or method step in the present disclosure isintended to be dedicated to the public regardless of whether theelement, component, or method step is explicitly recited in the claims.No claim element herein is to be construed under the provisions of 35U.S.C. 112, sixth paragraph, unless the element is expressly recitedusing the phrase “means for.”

What is claimed is:
 1. A method of making a light-redirecting fabricfrom a flexible sheet of optically clear material, comprising: a step offorming a parallel array of slits in a surface of said sheet by means ofat least one blade or razor; a step of elastic elongation of said sheetalong a direction perpendicular to said slits; and a step of making atleast a portion of said elongation irreversible.
 2. A method of making alight-redirecting fabric as recited in claim 1, wherein said step ofmaking at least a portion of said elongation irreversible includeselongation of said sheet in a plastic deformation mode.
 3. A method ofmaking a light-redirecting fabric as recited in claim 1, wherein saidstep of making at least a portion of said elongation irreversibleincludes annealing of said sheet at an elevated temperature.
 4. A methodof making a light-redirecting fabric as recited in claim 1, wherein saidstep of making at least a portion of said elongation irreversibleincludes heating at least a portion of said sheet so as to causematerial flow.
 5. A method of making a light-redirecting fabric asrecited in claim 1, wherein said step of making at least a portion ofsaid elongation irreversible includes heating at least a portion of saidsheet so as to exceed the yield strength of said sheet.
 6. A method ofmaking a light-redirecting fabric as recited in claim 1, wherein theirreversible portion of said elongation is less than 5% of the originalsheet length.
 7. A method of making a light-redirecting fabric asrecited in claim 1, wherein said blade or razor is of a rotary type. 8.A method of making a light-redirecting fabric as recited in claim 1,further comprising a step of heating at least a surface portion of saidsheet to a predetermined temperature.
 9. A method of making alight-redirecting fabric as recited in claim 1, further comprising astep of lamination of said sheet onto a surface.
 10. A method of makinga light-redirecting fabric, comprising: providing a sheet of soft,optically clear material; forming a parallel array of slits in a surfaceof said sheet using at least one blade or razor; and elongating saidsheet along a direction perpendicular to said slits by a predeterminedamount with respect to the original length.
 11. A method of making alight-redirecting fabric as recited in claim 10, wherein said blade orrazor is of a rotary type.
 12. A method of making a light-redirectingfabric as recited in claim 10, further comprising heating at least aportion of a surface of said sheet to a predetermined elevatedtemperature.
 13. A method of making a light-redirecting fabric asrecited in claim 10, further comprising a step of lamination of saidsheet onto a surface.
 14. A method of making reflective surfaces withina soft, optically transmissive material, comprising: slitting a surfaceof said material with a sharp object so as to produce a slit with a rootmean square surface profile roughness parameter between 10 nanometersand 100 nanometers.
 15. The method of claim 14, further comprisingheating said material at least in the area of said slitting.
 16. Themethod of claim 14, further comprising elongating said material in adirection perpendicular to the slitting direction.
 17. The method ofclaim 14, further comprising stretching said material in a directionperpendicular to the slitting direction.
 18. The method of claim 14,further comprising separating the walls of each formed slit by apredetermined distance.
 19. The method of claim 14, further comprisingproviding a space between the opposing walls of said slit and allowingthe ambient air to fill said space.
 20. The method of claim 14, whereinsaid sharp object is a rotary blade having a thickness of at most 0.3mm.